THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 


GIFT  OF 

Join  S.Prell 


GAS  ENGINE  THEORY 
AND  DESIGN 


A.    C.    MEHRTENS,    M.E. 

INSTRUCTOR   IN   MECHANICAL  ENGINEERING 

ENGINEERING  SCHOOL 
MICHIGAN  AGRICULTURAL  COLLEGE 


FIRST  EDITION 

FIRST   THOUSAND 


NEW  YORK 

JOHN  WILEY  &  SONS 

LONDON:    CHAPMAN  &  HALL,   LIMITED 

1909 


Copyright,  19(19 
BY  A.  C.  MEHRTENS 


Electrotyped  and  Printed  by  the  Publishers  Printing  Co.,  New  York,  U.  S.  A. 


Engineering 
Library 

TJ 

110 


CONTENTS 


* 

3 


CHAPTER  PAR. 

I.  GENERAL  PRINCIPLES  OF  OPERATION l-7a 

II.  HISTORICAL 8-14a 

III.  APPLICATIONS  OF  THE  GAS  ENGINE 15-15a 

IV.  HEAT — THERMODYNAMICS 16-336 

V.  COMBUSTION    34-55 

VI.  FUELS 56-72 

VII.  LAWS  OF  GASES 73-91 

VIII.  GAS-ENGINE  EFFICIENCY 92-97 

IX.  EXPLOSIVE  MIXTURES 98-104 

X.  MIXING  VALVES  AND  CARBURETERS 105-107 

XI.  GOVERNING 108-112 

XII.  IGNITION 113-123 

XIII.  COOLING 124-130 

XIV.  EXHAUST 131-134 

XV.  SELECTION  OF  TYPE 135-140 

XVI.  DETERMINATION  OF  THE  PRINCIPAL  DIMENSIONS..  .  .  141-147 

XVII.  FORCES  ACTING  IN  THE  GAS  ENGINE 148-158 

XVIII.  DESIGN  AND  DIMENSIONS  OF  PARTS 159-183 

XIX.  GAS-ENGINE  MANIPULATION 184-188 

XX.  TESTING 189-200 

XXI.  DESIGNS  .  .                                                                      .  201-202 


TABLES 


N} 


PAR. 

33a 

60a 

71a 

91 

146a 

^ 183 

' 147 

S.  VOLUME,  PRESSURE  AND  TEMPERATURE  CURVES 149 


1.  PHYSICAL  PROPERTIES  OF  MATERIALS 

2.  PETROLEUM  DISTILLATES 

3.  PROPERTIES  OF  FUEL  GASES 

4.  VOLUMES  AND  SPECIFIC  HEATS  OF  GASES. 

5.  EFFICIENCIES  AT  DIFFERENT  ALTITUDES.  .  , 

6.  STRENGTH  OF  MATERIALS 

7.  HEAT  AND  POWER  UNITS.  .  .  ...... 


713773 

Engineering 
Library 


PRELIMINARY 

IT  has  been  the  aim  of  the  author  to  prepare  a  book  for 
all  who  are  interested  in  gas  engines — students,  draughts- 
men, engineers,  as  well  as  the  men  who  operate  gas  engines 
of  any  kind,  and  wish  to  become  better  acquainted  with 
the  theory  and  the  why  of  many  things. 

The  book  should  be  of  special  interest  to  the  technical 
student,  and  was,  in  fact,  first  prepared  for  the  engineering 
classes  at  the  Michigan  Agricultural  College,  since  no  suit- 
able text-book  could  be  found. 

The  reading-matter  throughout  has  been  arranged  care- 
fully and  with  a  definite  object  in  view.  The  large  number 
of  figures  illustrating  the  text  have  been  made  as  simple 
as  possible.  It  has  also  been  the  aim  of  the  author  to  make 
the  treatment  clear  and  concise,  and  for  this  reason  every 
paragraph  should  be  studied — not  merely  read  over. 

It  is  hoped  that  this  book  will  enable  every  earnest  stu- 
dent to  acquire  a  foundation  upon  which  he  may  eventually 
build  a  broad  and  comprehensive  knowledge  of  the  subject. 

Acknowledgment  is  due  Professor  L.  L.  Appleyard  for 
his  kindly  criticism  and  assistance  in  reading  the  proofs. 


GAS-ENGINE  THEORY 
AND  DESIGN 

CHAPTER  I 

(SKXKRAL    PRINCIPLES    OF   OPERATION 

1.  THE  HEAT  ENGINE  may  be  defined  as  a  machine  which 
converts  heat  into  mechanical  energy.    The  heat  sets  the 
engine  in  motion  and  the  engine,  by  virtue  of  this  motion, 
can  drive  machinery  to  which  it  is  connected.     The  two 
principal  classes  of  heat  engine  are  the  Steam  Engine  and 
the  Gas  Engine. 

2.  THE  GAS  ENGINE,  or  internal-combustion  engine,  is 
a  machine  in  which  the  fuel  is  burned  directly  in  the  engine 
cylinder.     Every  internal-combustion  engine  is  a  gas  en- 
gine, no  matter  whether  the  fuel  applied  is  a  gas  or  a  liquid, 
since,  in  the  act  of  burning,  a  liquid  fuel  is  first  converted 
into  a  gas. 

3.  A   GENERAL   CLASSIFICATION    of   gas  engines   is   as 
follows : 

(a)  According  to  the  fuel :  Gas  engines,  operating  on  fuel 
in  the  form  of  gas;  oil  engines,  engines  operating  on  fuel 
in  the  form  of  oils  heavier  than  gasolene — such  as  kerosene, 
fuel  oil,  crude  oil;  gasolene  engines,  engines  operating  on 
gasolene;  alcohol  engines,  engines  operating  on  alcohol. 

(6)  According  to  the  R. P.M.  (revolutions  per  minute): 
High  speed,  for  example,  an  automobile  engine  running  at 
i 


GAS-ENGINE  THEORY   AND   DESIGN 


FIG.   1. 


1,2(M)  R.P.M.;  slow  speed,  a  stationary  engine  running  at 
KM)  R.P.M. 

The  piston  speed  in  feet  per  minute  may  be  the  same 
for  both  a  high-speed  and  a  slow-speed  engine.    This  can 
readily  be  seen  by  assuming  that  the  automobile  engine 
mentioned    above    has    a   4" 
stroke  and  the  stationary  en- 
gine a  48"  stroke. 

(c)  According  to  the  strokes 
required  to  complete  a  working 
cycle:  Two-stroke  cycle;  four- 
stroke  cycle. 

Gas  engines  can  also  be  classified  according  to  the  me- 
chanical construction  as  single-cylinder,  multiple-cylinder, 
horizontal,  vertical,  etc. 

4.  A  CYCLE  is  a  complete  succession  of  events,  or  chain 
of  events.     The  following  paragraphs  will  make  clear  the 
term  " cycle"  as  applied  in  this  chapter. 

5.  GENERAL  PRINCIPLES  OF  OPERATION. — In  Fig.  1  we 
have  a  cylinder  C,  a  piston  P  which  can  readily  lxi  pushed 
back  and  forth,  but  fits  so  closely  to  the  cylinder  that  air 
cannot  leak  past.    The  space  A  is 

filled  with  a  mixture  of  air  and 
fuel  gas,  or  air  and  the  vapor  of 
some  liquid  fuel,  such  as  gasolene. 
This  mixture  is  at  atmospheric 
pressure  and  when  ignited  the  re- 
sulting combustion  raises  the  tem- 
perature, and  consequently  the  pressure  in  A,  causing  the 
mixture  to  expand  and  push  the  piston  out  to  the  position 
indicated  by  the  dotted  lines,  or  until  the  pressure  again 
drops  to  atmospheric. 

Now,  it  has  been  found  that  if  the  mixture  of  air  and 
fuel  is  first  compressed  by  moving  the  piston  in,  as  shown 


GENERAL   PRINCIPLES     OF     OPERATION 


•W 


in  Fig.  2,  and  then  ignited,  the  piston  will  be  driven  out 
with  much  greater  force  than  before  and  more  work  is 
gotten  out  of  the  same  amount  of  fuel.  All  gas  engines 
to-day  work  on  the  compression  principle.  The  matters 
of  compression,  ignition,  combustion,  etc.,  will  be  consid- 
ered more  fully  later  on. 

6.  THE  TWO-STROKE  CYCLE. — The  next  four  figures  illus- 
trate the  operation  of  the  two-cycle  engine.  Fig.  3  shows 
the  piston  P  ready  to  start 
on  its  down  stroke.  The 
combustion  chamber  S  is 
filled  with  a  compressed 
mixture  of  air  and  com- 
bustible which  has  been 
compressed  by  the  preced- 
ing up  stroke  of  the  pis- 
ton. This  mixture  is  now 
ignited  (usually  by  means 
of  an  electric  spark)  and 
the  expansion  of  the  burn- 
ing gases  drives  the  piston 
down.  As  P  nears  the  end 
of  its  down  stroke  it  un- 
covers the  exhaust  port  E 
through  which  the  burnt 
gases  flow  from  the  cylin- 
der. A  little  later  the  inlet  port  7,  leading  from  the 
crank  chamber  to  the  cylinder,  is  uncovered  by  P.  The 
crank  chamber  C  is  air-tight,  and  in  moving  down  P  covers 
up  the  air-inlet  port  A  and  compresses  the  combustible 
charge  in  C.  When  /  is  uncovered  by  P  this  compressed 
charge  flows  from  C  into  the  cylinder,  partly  filling  it  with 
a  fresh  combustible  charge  and  helping  to  expel  some  of 
the  remaining  burnt  gases.  The  baffle  plate  B  deflects  the 


FIG.  3. 


4  GAS-ENGINE  THEORY  AND   DESIGN 

incoming  charge,  as  shown  by  the  arrows,  preventing  it 
from  passing  directly  across  the  cylinder  and  out  through 
the  exhaust  port. 

Fig.  4  shows  the  piston  ready  to  start  on  its  up  stroke. 
In  moving  up  P  covers  A,  E,  and  /,  and  compresses  the 

charge  in  the  cylinder. 
As  soon  as  A  is  uncovered 
the  motion  of  P  sucks  a 
charge  of  air  and  combus- 
tible into  C  and  this  is 
again  compressed  on  the 
next  down  stroke.  The 
air  and  combustible  must 
be  thoroughly  mixed  be- 
fore passing  into  C.  \Yhen 
P  again  reaches  the  posi- 
tion shown  in  Fig.  3  it  has 
completed  a  working  cycle 
in  two  strokes,  or  one  rev- 
olution. Ignition  now  oc- 
curs and  the  foregoing 
operations  are  repeated. 
The  piston  is  kept  in  mo- 
tion during  the  time  it  receives  no  working  impulse  by 
means  of  a  heavy  fly-wheel. 

The  circle  in  Fig.  ">  represents  the  travel  of  the  crank-pin 
D,  shows  what  takes  place  above  the  piston  and  approx- 
imately what  part  of  a  revolution  is  required  for  the 
various  operations. 

Fig.  6  shows  what  takes  place  in  the  crank  chamber 
during  one  revolution. 

In  large  two-cycle  gas  engines  the  preliminary  compres- 
sion of  air  and  fuel  takes  place  in  separate  pumps  Instead 
of  in  a  closed  crank  chamber. 


FIG.  4. 


GENERAL   PRINCIPLES   OF  OPERATION 


The  space  W  is  filled  with  water  which  circulates  through 
the  jacket  and  absorbs  and  carries  away  surplus  heat  so 
as  to  prevent  the  cylinder  from  overheating,  since  the  tem- 


FIG.  5. 

perature  of  combustion  is  veiy  high.    The  water  is  usually 
circulated  by  means  of  a  pump. 

A  two-cycle  engine  of  the  type  illustrated  can  run  in 
either  direction. 

The  direction  of  rotation  is  indicated  by  the  arrows. 

7.  THE  FOUR-STROKE  CYCLE. — The 
next  six  figures  illustrate  the  operation 
of  the  four-cycle  engine.  Fig.  7  shows 
the  piston  ready  to  start  on  its  down 
stroke.  The  air-inlet  valve  V  is  open, 
the  exhaust  valve  V  is  closed;  as  the 
piston  moves  down  it  draws  a  charge  of 
air  and  combustible  into  the  cylinder. 

Fig.  8  shows  P  ready  to  start  on 
its  up  stroke.  Both  valves  are  closed 
and  the  charge  in  S  is  compressed  as 
P  moves  up. 

Fig.  9  shows  P  ready  to  start  on  its 
second  down  stroke.  Both  valves  are  FIG.  7. 


6  GAS-ENGINE  THEORY   AND   DESIGN 

closed,  the  compressed  charge  is  ignited,  and  the  resulting 
expansion  drives  P  down. 

Fig.  10  shows  P  ready  to  start  on  its  second  up  stroke. 
V  is  closed,  V  is  open,  and  as  P  moves  up  it  drives  the 
burnt  gases  from  the  cylinder. 

One  working  cycle  has  now  been  completed  in  four  piston 
strokes,  or  two  revolutions,  and  this  cycle  is  repeated  in- 
definitely until  the  engine  stops  running. 


FIG.  8. 


FIG.  9. 


FIG.  10. 


Figs.  11  and  12  show  approximately  during  what  periods 
of  the  crank-pin  travel  these  operations  take  place. 

The  four-cycle  engine  will  run  in  one  direction  only  with 
the  valve  gearing  arranged  in  the  ordinary  manner. 

la.  RESUME. — The  student  has  learned  from  the  fore- 
going that:  In  the  two-cycle  engine  there  is  one  power 
impulse  for  every  revolution  of  the  crank-pin ;  in  the  four- 
cycle engine  there  is  one  power  impulse  for  every  two 
revolutions  of  the  crank-pin ;  the  operation  of  the  gas  engine 
consists  of; 


GENERAL   PRINCIPLES  OF   OPERATION 


(a)  Causing  a  charge  of  air  and  combustible  to  flow  into 
the  engine  cylinder; 

(6)  Compressing  this  charge; 

(c)  Igniting  the  compressed  charge  and  driving  the  piston 
out  by  means  of  the  expansion  of  the  highly  heated  gases; 

(d)  Discharging  the  burnt  gases  from  the  cylinder. 


FIG.  11. 


FIG.  12. 


Also,  that  a  gas  engine  must  be  provided  with  means 
for  mixing  the  air  and  combustible  before  the  charge  passes 
into  the  cylinder; 

That  means  must  be  provided  for  igniting  the  combus- 
tible charge  at  the  end  of  the  compression  stroke; 

That  the  cylinder  wall  must  be  water-cooled  to  prevent 
overheating ; 

That  a  fly-wheel  must  be  provided  to  keep  the  engine 
running  during  the  idle  strokes  and  furnish  the  necessary 
power  for  compressing  the  combustible  charge. 

In  the  four-cycle  engine  the  valves  are  opened  and  closed 
at  the  proper  time  by  springs  and  cams  operated  by  gearing 
driven  from  the  crank-shaft. 


CHAPTER  II 

HISTORICAL 

8.  Very  little  is  known  of  early  attempts  to  produce  a 
heat  engine,  and  it  is  doubtful  if  such  machines  were  ever 
constructed  in  ancient  times.  Several  nations  attained  a 
high  degree  of  civilization  many  centuries  ago,  and  now 
and  then  the  investigator  comes  across  a  toy  set  in  motion 
by  steam  or  hot  air,  but  there  seems  to  be  no  record  of  a 
machine  powerful  enough  to  do  work.  In  1774  James  Watt, 
in  England,  completed  the  first  commercially  successful 
steam  engine.  This  was  applied  to  the  driving  of  machin- 
ery, pumping,  etc.  After  a  while  locomotives  were  devel- 
oped and  the  steam  engine  was  applied  to  navigation. 

It  early  occurred  to  investigators  that  if  fuel  could  be 
burned  directly  in  the  engine  cylinder  a  great  deal  of  the 
heat  loss,  which  occurs  in  the  roundabout  way  of  applying 
heat  in  the  steam  engine,  could  be  avoided.  The  simplicity 
and  economy  of  a  first-class  gas  engine  as  compared  to 
the  steam-power  plant  bears  out  the  correctness  of  these 
early  ideas.  The  first  successful  type  of  internal-combustion 
engine  was  the  gas  engine  proper,  i.e.,  one  using  gas  as  fuel. 
The  manufacture  of  gas  from  coal  for  illuminating  purposes 
had  become  fairly  well  established  by  the  middle  of  the 
nineteenth  century.  In  1804  the  Lyceum  Theatre,  in 
London,  was  illuminated  by  gas,  and  in  1810  the  first  public 
gas-lighting  plant  was  installed.  In  1823  gas-lighting  was 
introduced  into  New  York  City.  As  time  went  on  improve- 
ments in  the  manner  of  manufacturing  gas  from  coal,  cul- 


HISTORICAL  9 

minating  in  the  gas-producer  of  to-day,  were  made.  In 
1859  petroleum  was  discovered  in  the  United  States  in 
great  quantities,  and  conditions  became  ripe  for  the  devel- 
opment of  the  internal-combustion  engine,  since  suitable 
fuels  (gas  and  oil)  were  now  at  hand.  The  great  advance 
made  in  America  in  refining  mineral  oil  stirred  up  Russia, 
the  next  largest  producer  of  petroleum  in  the  world,  and 
American  and  Russian  oils  were  soon  carried  to  all  parts 
of  the  globe.  All  the  various  attempts  to  produce  gas  en- 
gines of  which  we  have  a  record  will  not  be  given  here,  but 
only  some  of  the  most  important  investigators  mentioned. 

9.  THE  LEXOIR  CYCLE. — In  1860  some  work  was  done  on 
the  gas  engine  by  Lenoir  in  France.    In  the  Lenoir  engine 
air  and  gas  were  drawn  into  the  cylinder  during  a  part 
of  the  suction  stroke,  the  inlet  valve  was  then  closed  and 
the  mixture  fired,  the  resulting  expansion  of  the  burning 
gases  driving  the  piston  out  during  the  remainder  of  the 
stroke.    During  the  return  stroke  the  cylinder  was  cleared 
of  the  burnt  gases.    This  was  a  non-compression  engine. 

10.  THE  BRAYTON  CYCLE.- — In  1873  Brayton,  in  America, 
brought  out  an  engine  in  which  combustion  took  place 
without  a  rise  in  pressure.    Air  and  gas  were  compressed  by 
means  of  a  pump  into  a  reservoir  which  communicated  with 
the  engine  cylinder.     During  about  one-half  of  the  out 
stroke  of  the  piston  the  charge  was  allowed  to  flow  into  the 
engine  cylinder  from  the  reservoir,  and  then  it  was  ignited. 
Wire  netting  prevented  the  flame  from  going  back  into  the 
reservoir.   At  about  half  stroke  the  reservoir  was  cut  off  and 
the  stroke  completed  by  the  expansive  working  of  the  gas. 

1 1 .  THE  BEAU  DE  ROCHAS  CYCLE. — The  compression  cycle, 
on  which  the  present-day  engines  operate,  was  first  sug- 
gested by  Beau  de  Rochas,  in  France,  in  1862.    In  1877, 
after  many  years  of  hard  work,  Dr.  N.  A.  Otto,  in  Germany, 
brought  out  the  first  commercially  successful  gas  engine 


10  GAS-ENGINE  THEORY   AND   DESIGN 

operating  on  the  Beau  de  Rochas  cycle.  This  engine  was 
a  great  improvement  upon  the  attempts  of  his  predecessors, 
and  the  development  of  the  gas  engine  from  now  on  was 
rapid.  This  cycle  (see  "Two-"  and  "Four-Cycle"  in  the 
preceding  chapter)  is  often  called  the  "Otto  Cycle." 

12.  In  1880  Dugald  Clerk,  in  England,  brought  out  the 
first  successful  "two-cycle"  engine  operating  on  the  Beau 
de  Rochas  cycle. 

Engines  working  on  fuels  other  tnan  gas  now  followed. 

13.  In  1884  Daimler,  in  Germany,  brought  out  a  light- 
weight, high-speed  oil  engine.     In  1895  he  introduced  a 
perfected  light-weight,  high-speed  gasolene  engine  suitable 
for  automobiles  and  motor-boats.    This  engine  was  quickly 
adopted  by  the  European    manufacturers,  especially  in 
France,  and  gave  a  great  impetus  to  the  development  of 
automobiling  and  motor-boating — in  fact,  made  the  pres- 
ent development  of  these  industries  possible.     The  im- 
provements made  in  carbureter  design  and  electric  ignition 
were  also  important  factors  in  this  connection. 

14.  THE    DIESEL  CYCLE. — Another  great   step   in   the 
development  of  the  gas  engine  was  made  in  1897,  when 
Rudolf  Diesel,  a  German  engineer,  brought  out  an  internal- 
combustion    engine    which    surpassed    all    previous    heat 
engines  in  the  matter  of  fuel  economy,  chiefly  by  means 
of  very  high  compression. 

The  Diesel  cycle  is  really  the  ordinary  compression  cycle, 
but  differs  from  it  in  the  manner  of  handling  the  fuel.  A 
charge  of  air  alone  is  compressed  to  500  or  600  Ibs.  per 
square  inch.  The  fuel  (oil)  is  injected  at  the  end  of  the 
compression  stroke  by  means  of  air  compressed  to  about 
800  Ibs.  in  a  small  two-stage  compressor.  The  fuel  is  in- 
jected from  the  time  the  piston  has  passed  the  dead  centre 
until  it  has  completed  about  one-tenth  of  the  expansion 
stroke  so  that  there  is  a  gradual  combustion,  and  the  com- 


HISTORICAL  11 

bustion  does  not  increase  the  pressure  in  the  cylinder,  but 
the  pressure  drops  throughout  the  expansion  stroke.  The 
temperature  at  the  completion  of  the  compression  stroke 
is  1000°  or  more,  so  that  the  oil  is  burned  directly  it  is  in- 
jected and  without  the  aid  of  any  ignition  apparatus. 
When  handled  in  this  manner  petroleum  and  its  distillates 
can  be  burned  completely  without  carbonizing. 

14a.  More  work  has  been  done  on  large  gas  engines 
in  Europe  than  here,  since  fuel  economy  is  of  greater  im- 
portance there.  The  first  1000-H.-P.  gas  engine  was  built 
in  1898  by  the  J.  Cockerill  Co.,  of  Seraing,  Belgium.  At 
that  time  the  building  of  such  a  large  gas  engine  was  looked 
upon  as  a  very  doubtful  venture.  To-day  gas  engines  are 
in  operation  developing  over  5000  H.-P.  in  single  units. 

One  great  factor  in  the  development  of  the  gas  engine 
has  been  the  utilization  of  blast-furnace  gas.  It  was  found 
that,  by  increasing  the  compression,  gases  that  are  very 
poor  in  quality  could  be  burned  in  the  gas  engine,  and  mil- 
lions of  cubic  feet  of  blast  furnace  and  coke-oven  gas  which 
were  formerly  wasted  at  the  great  steel  works  are  now 
converted  into  power  by  means  of  the  gas  engine. 

Automobiles  and  motor-boats  have  practically  been 
developed  during  the  past  twelve  years,  and  large  gas 
engines  during  the  past  nine  years.  The  smaller  com- 
mercial gas  engine  is  barely  thirty  years  old.  The  growth 
of  the  gas-engine  industry  has  increased  at  a  tremendous 
pace  in  the  past  few  years,  and  a  few  figures  may  prove 
interesting:  In  1881  the  gas  engines  in  200  European  cities 
aggregated  a  total  of  2,442  H.-P.,  and  in  1902  this  had 
reached  a  total  of  123,000  H.-P.  The  total  of  large  gas 
engines  in  operation  in  Germany  at  the  present  time  ex- 
ceeds 400,000  H.-P. 

The  present  gas-engine  power  of  the  world  has  been 
estimated  as  about  4,000,000  H.-P. 


CHAPTER  III 

APPLICATIONS   OF   THE   GAS   ENGINE 

1").  A  brief  review  of  the  principal  applications  and  of 
some  of  the  large  gas-power  installations  in  this  country 
may  prove  interesting. 

In  1896  the  Westinghouse  Machine  Company  put  on  the 
market  a  vertical  multiple-cylinder  gas  engine  suitable  for 
driving  generators  and  general  power  work.  Up  to  the 
present  time  this  company  has  installed  gas  engines  aggre- 
gating about  100,000  H.-P. 

The  DC  La  Vergne  Machine  Company  has  installed  at 
the  Lackawanna  Steel  Company's  plant  two-cycle  Koerting 
gas  engines  operating  on  blast-furnace  gas  and  aggregating 
43,200  H.-P.  This  company  has  also  installed  at  the  Bald- 
win Locomotive  Works  oil  engines,  operating  on  cheap  fuel 
oils,  aggregating  5,315  H.-P. 

The  Allis-Chalmers  Company  has  under  construction  at 
the  time  of  writing  the  following  large  units:  25  2,000 
K.W.  units  direct-connected  to  generators  (about  70,000 
H.-P.);  12  3,000-H.-P.  blowers;  9  1,000  K.W.  units  con- 
nected to  generators. 

A  number  of  the  above  units  are  for  the  Indiana  Steel 
Company,  which  will  install  in  its  plant  at  Gary,  Indiana, 
gas  engines  aggregating  over  100,000  H.-P.  Gas  power  will 
be  used  for  driving  generators  and  the  electric  current  will 
be  used  for  driving  rolling-mill  and  other  machinery.  Gas 
power  will  also  be  used  for  operating  the  blowers. 

The  San  Mateo  Power  Company,  at  its  Martin  Station, 
12 


'/ 


14  r,AS-KNY5IXE  THEORY    AND    DESIGN 

California,  has  in  operation  three  gas-power  units  driving 
generators,  and  two  more  units  will  be  installed.  These 
engines  were  built  by  the  Snow  Steam  Pump  Works, 
operate  on  gas  made  from  crude  oil  by  the  Lowe  process, 
and  have  a  maximum  capacity  of  about  5,300  H.-P.  each. 

Besides  the  foregoing  there  are,  of  course,  a  great  num- 
IXT  of  smaller  installations  of  gas  engines  driving  generators, 
pumps,  mill  and  factory  machinery,  etc.  For  small  station- 
ary power  purposes  the  gas  engine  has  the  field  to  itself. 

Not  so  many  years  ago  there  were  practically  no  auto- 
mobiles in  the  United  States.  According  to  statistics  that 
have  been  compiled  the  total  value  of  pleasure  automobiles 
which  have  been  produced  in  the  United  States  in  1907 
was  $105,669,572.  The  capital  invested  that  year  in  the 
industry  was  §94,200,000,  and  the  number  of  employees 
58,000.  It  is  estimated  that  the  number  of  employees  in 
factories  turning  out  automobile  accessories  was  29,000, 
and  the  capital  invested  §36,700,000.  There  were  2,151 
sales  offices  and  garages  which  employed  21,500  persons 
and  the  capital  invested  in  these  garages  wras  §57,500,000. 
Over  40,000  automobiles  have  been  registered  in  New  York 
State,  and  of  these  about  25,000  are  in  New  York  City. 

The  motor-cycle  industry  is  rapidly  growing.  The  fastest 
mile  ever  made  by  man,  viz.,  in  26f  seconds,  was  done  on 
a  racing  motor-cycle. 

.  In  this  country  the  number  of  business  and  pleasure  boats 
propelled  by  gasolene  and  oil  engines  has  been  placed  at 
400,000,  and  these  are  increasing  at  the  rate  of  75,000  a  year. 
The  value  of  the  marine  gas  engine  has  been  recognized  by 
several  governments,  and  the  use  of  motor-boats  for  general 
government  and  naval  service  is  constantly  increasing. 
The  Imperial  Russian  Marine  has  in  service  ten  torpedo 
boats,each  propelled  by  gasolene  motors  of  600H.-P.  These 
torpedo  boats  are  capable  of  a  speed  of  21  knots  (24.2 


1 


16  GAS-ENGINE  THEORY   AND   DESIGN 

miles)  an  hour.  Several  governments  have  in  operation 
gasolene-propelled  submarine  boats. 

In  190.")  the  Gregory,  a  motor-boat  91  feet  long  and 
equipped  with  two  300  H.-P.  Standard  engines,  crossed  the 
Atlantic — the  first  gas-power  boat  to  perform  this  feat. 
At  the  present  time  there  arc1  several  motor-boats  that  can 
do  close  to  30  miles  an  hour. 

The  gas  engine,  in  connection  with  the  gas-producer, 
will  doubtless  be  extensively  applied  to  navigation  in  the 
near  future1.  Several  beginnings  have  already  been  made1 
in  this  direction  both  here  and  abroad.  A  gas-producer 
occupies  about  one-fourth  of  the  space  of  a  water-tube 
boiler,  and  the  coal  consumption  is  about  one-third.  There 
is  consequently  a  great  saving  in  coal  space1,  boiler  space, 
and  cost  of  running.  In  one  instance  the  cost  of  carrying 
freight  by  a  gas  boat  was  only  one-fifth  of  that  of  transport- 
ing it  by  rail. 

The  oil  engine  is  being  applied  more  and  more  to  general 
portable,  contracting,  and  agricultural  work. 

Thc>  oil  traction-engine  is  used  for  plowing,  threshing, 
pumping,  anel  hauling  the  produce  of  the  farm  to  the  market 
over  considerable  distances.  With  power-driven  machinery 
a  few  me1!!  can  take  care  of  a  thousand  acres  of  wheat  land. 

15a.  Gasolene  motor-cars  are  being  used  by  several  rail- 
mads  for  short-run  traffic.  The  Union  Pacific  has  9  of  these 
cars  in  operation  at  the  present  time,  and  is  building  22  more. 
One  of  the  cars  is  5.5  feet  long,  has  a  seating  capacity  of  75, 
is  equipped  with  a  200  H.-P.  6-cylinder  gasolene  engine,  and 
can  easily  make  65  miles  an  hour.  The  car  can  be  starteel 
and  stopped  quicker  than  an  electric  car.  The  cost  of  oper- 
ation is  below  that  of  either  steam  or  electric  power. 

Gasolene  engines  weighing  about  two  and  a  half  pounds 
per  H.-P.  have  been  constructed  for  flying-machine  and 
racing  purposes. 


APPLICATIONS  OF  THE  GAS  ENGINE 


17 


18 


GAS-ENGINE  THEORY  AND   DESIGN 


APPLICATIONS   OF  THE   GAS   ENGINE 


19 


The  difference  in  design  between  a  high-speed  engine 
and  a  slow-speed  stationary  engine  is  strikingly  shown  by 
their  respective  weights.  Many  stationary  gas  engines 
average  500  Ibs.  per  H.-P.,  and  even  more,  while  an  auto- 


FIG.  17.     Warren  Gas  Engine. 


20  GAS-ENGINE  THEORY  AND   DESIGN 

mobile  engine  averages  10  Ibs.  per  H.P.  A  50-H.-P.  engine 
of  the  type  first  mentioned  would  weigh  25,000  Ibs.,  while 
the  automobile  engine  would  weigh  500  Ibs. 

A  word  might  here  be  said  about  the  cost  of  operation. 
This  depends  largely  upon  local  conditions  which  determine 
the  kind  and  cost  of  the  fuel  to  be  used.  The  author  has 
in  mind  an  instance  where  some  oil  engines  consumed  two- 
thirds  of  a  pint  of  crude  oil  per  H.-P.  hour.  The  oil  was 
supplied  at  a  net  cost  of  2  cents  per  gallon.  The  cost  of 
fuel  per  H.-P.  hour  was  therefore  one-sixth  of  a  cent. 

In  conclusion,  it  might  be  mentioned  that  there  are  at 
the  present  time  over  300  builders  of  gas  engines  in  this 
country. 

Fig.  13  shows  an  arrangement  which  is  now  the  standard 
for  the  large  Allis-Chalmers  gas  engines.  The  engine  illus- 
trated is  a  four-cycle  doublet-acting  twin  tandem  machine 
driving  a  2,500  K.W.  generator.  The  maximum  rating  of 
the  engine  on  producer  gas  is  4,500  H.-P.,  running  at  83 
R.P.M.  The  floor  space  occupied  is  69  ft.  by  35  ft.  The 
cylinder  dimensions  are  44"  x  54"  stroke.  The  diameter  of 
the  crank-shaft  is  30",  and  the  crank-pin  20".  The  length 
of  the  main  bearing  is  54".  The  flywheel  is  23  ft.  in  diam- 
eter, and  the  weight  varies  according  to  conditions.  The 
weight  of  the  main  frame  is  90  tons. 

Figs.  15,  16,  and  17  show  clearly  the  constructive  details 
of  the  Warren  heavy-duty  tandem  gas  engine  built  by  the 
Struthers  Wells  Co. 

Fig.  18  is  a  photographic  reproduction  of  a  90-H.-P.  ver- 
tical engine  and  100-H.-P.  producer  built  by  this  company. 
The  general  over-all  dimensions  of  the  engine  are:  Height, 
9'0";  length  HXO";  width  7'0".  The  producer  occupies  a 
floor  space  about  9'  square,  and  the  highest  point  is  14' 
from  the  ground. 

Some  tests  on  Warren  engines  are  given  under  "Testing." 


CHAPTER   IV 

HKAT.       THERMODYNAMICS 

16.  Heat    may  be  defined  as  a  form  of  energy   which 
enables  us  to  do  work. 

17.  THEORY  OF  HKAT. — The  sensation  of  heat  is-supposed 
to  be  caused  by  the  rapid  vibration  of  the  molecules  of 
a  body.    The  hotter  a  body  the  more  rapid  these  vibrations, 
and  the  colder  the  body  the  less  rapid  the  vibrations. 

IS.  EXPAXSIOX. — The  terms  "heat"  and  "expansion" 
are  practically  synonymous.  That  heat  will  expand  the 
things  that  are  heated  is  one  of  the  most  patent  facts  in 
our  every-day  life.  When  we  heat  one  end  of  an  iron  bar 
to  a  red,  or  white,  heat,  the  hot  end  is  larger  than  the  cold 
one  and  the  difference  in  size  is  plainly  visible.  When 
water  is  heated  it  expands.  When  air  is  heated  it  expands 
and  occupies  more  space  than  before,  weighing  less,  of 
course,  than  the  same  bulk  of  cold  air.  We  here  have 
examples  of  a  solid,  a  liquid,  and  a  gas  expanding  through 
heating,  and  this  increase  in  volume  due  to  heating  is  taken 
advantage  of  in  the  heat  engine  to  convert  heat  energy  into 
mechanical  energy — it  is  the  fundamental  principle  of  the 
working  of  all  heat  engines. 

10.  TEMPERATURE. — When  one  body  is  at  a  higher  tem- 
perature than  another,  i.e.,  when  it  is  warmer,  heat  tends 
to  flow  from  the  warm  body  to  the  cold  one  until  both  are 
at  the  same  temperature.  When  hot  water,  for  example,  is 
poured  into  a  vessel  containing  cold  water  an  interchange 
of  heat  takes  place,  the  hot  water  loses  some  of  its  heat 

22 


HEAT.    THERMODYNAMICS  23 

and  the  cold  water  absorbs  it,  and  this  interchange  goes 
on  until  all  the  water  is  at  the  same  temperature.  We  find 
this  tendency  toward  an  equilibrium  of  heat  on  every  hand, 
it  is  universal,  and  it  is  of  the  greatest  importance  in  the 
working  of  the  heat  engine.  Those  materials  that  feel  cold 
to  the  touch,  like  metals,  simply  conduct  the  heat  away 
more  rapidly  than  other  materials.  Temperature  is  an 
indication  of  the  intensity  and  not  of  the  amount  of  heat. 
A  boiler  may  contain  water  at  a  temperature  of  200°,  and 
the  temperature  of  a  blow-pipe  flame  may  be  2000°,  but  the 
amount  of  heat  in  the  boiler  may  be  several  thousand 
times  that  of  the  blow-pipe  flame.  Temperature  depends 
upon  the  rate  of  combustion  and  not  upon  the  total 
amount  of  heat. 

20.  SOURCES  OF  HEAT. — Heat  is  obtained  in  various 
ways — from  the  sun,  by  the  combustion  of  fuels,  by  means 
of  the  electric  current,  friction,  percussion,  etc.    The  heat 
used  in  the  various  heat  engines  is  invariably  obtained  from 
the  combustion  of  a  fuel. 

21.  TRANSFER  OF  HEAT. — Heat  is  transferred  by  con- 
duction, radiation,  and  convection. 

(a)  Conduction  is  the  transfer  of  heat  in  a  body  from  a 
higher  to  a  lower  temperature.  When  one  end  of  an  iron 
bar  is  heated  some  of  the  heat  is  transferred  along  the  bar 
by  conduction. 

(6)  Radiation  is  the  transfer  of  heat  from  a  hot  body 
to  a  colder  one  across  an  intervening  space.  The  sun 
heating  the  earth  is  an  example  of  this.  The  intervening 
medium  may  not  necessarily  become  heated. 

(c)  Convection  is  the  transfer  of  heat  by  the  motion  of 
the  heated  matter.  Liquids  and  gases  heated  from  below 
are  examples  of  this.  The  hot  currents  of  fluid  rise  and 
warm  the  surrounding  fluid,  the  colder  fluid  sinks  to  the 
bottom  of  the  containing  vessel,  and  upon  becoming 


24  GAS-ENGINE  THEORY   AND   DESIGN 

heated  rises  to  the  top.  When  heated  from  above  liquids 
and  gases  are  very  poor  conductors  and  cannot  heat  by 
convection. 

22.  RADIATION  OF  HEAT. — The  intensity  of  heat  radiated 
from  any  source  varies,  as: 

The  temperature  of  the  source ; 

Inversely,  as  the  square  of  the  distance  from  the 
source ; 

Changes,  as  the  inclination  of  rays  to  the  surface 
changes. 

A  polished  surface  will  give  out  less  heat,  and  absorb 
less  heat,  than  a  non-polished  surface. 

It  is  sometimes  very  important  that  the  radiation  of  heat 
be  prevented  as  much  as  possible,  as  in  the  case  of  a  steam- 
engine  cylinder,  and  the  cylinder  is  then  covered  with 
some  material  that  is  a  poor  conductor  of  heat.  Among 
the  best  materials  in  common  use  for  this  purpose  are 
mineral  wool  and  asbestos.  Air  is  a  poor  conductor  of 
heat  when  it  has  no  chance  to  circulate,  and  for  this  reason 
a  material  of  a  woolly  nature  will  constitute  a  good  non- 
conductor, provided  it  will  withstand  the  required  tem- 
perature. 

23.  HEAT  UXIT. — A  heat  unit  is  the  amount  of  heat 
required  to  raise  the  temperature  of  a  pound  of  water  1°. 
This  is  termed  a  British  Thermal  Unit,  or  B.T.U. 

24.  MECHANICAL  EQUIVALENT  OF  HEAT. — Joule's  experi- 
ments (1843-50)  proved  that  heat  and  mechanical  energy 
are  mutually  convertible  and  that  there  is  a  constant  and 
definite  relation  between  the  amount  of  work  that  can  be 
done  by  a  certain  amount  of  heat,  and  vice  versa.    Joule 
placed  some  paddles  in  a  vessel  filled  with  water  in  such  a 
manner  that  the  falling  of  a  weight  revolved  the  paddles 
and  the  friction  caused  by  the  motion  of  the  paddles  raised 
the  temperature  of  the  water.     From  these  and  later  ex- 


HEAT.    THERMODYNAMICS  25 

periments  the  value  of  the  B.T.U.  was  established — which 
is  now  generally  accepted  as  778  foot-pounds  (a  foot- 
pound is  the  work  done  in  lifting  vertically  1  pound  1  foot), 
i.e.,  it  requires  778  foot-pounds  of  work  to  raise  the  tem- 
perature of  1  pound  of  water  1°. 

25.  SENSIBLE  AND  INSENSIBLE  HEAT. — Sensible  heat  is 
that  which  can  be  perceived  by  the  senses.     Insensible 
heat  is  that  which  cannot  be  perceived  by  the  senses.    The 
latent  heat  of  water  is  an  example  of  the  latter. 

26.  SPECIFIC    HEAT. — Different    bodies    have    different 
capacities  for  storing  heat.    Specific  heat  is  the  amount  of 
heat  required  to  raise  the  temperature  of  a  unit  mass  of  a 
body  1°  as  compared  with  some  standard.    The  amount  of 
heat  required  to  raise  the  temperature  of  1  pound  of  water 
1°  from  32°  F.  (the  freezing-point  of  water)  is  taken  as  the 
standard.     For  high  temperatures  the  specific  heats  are 
somewhat   greater    than    the   values  given   in  Tables  I 
and   IV. 

27.  LATENT  HEAT. — Latent  heat  is  the  extra  amount  of 
heat  necessary  to  force  the  molecules  of  a  body  farther 
apart,  and  overcome  the  forces  of  cohesion,  in  order  to 
change  the  state  of  the  body. 

28.  LATENT  HEAT  OF  FUSION. — This  is  the  extra  amount 
of  heat  necessary  to  convert  a  solid  into  a  liquid. 

29.  LATENT  HEAT  OF  VAPORIZATION. — This  is  the  extra 
amount  of  heat  necessary  to  convert  a  liquid  into  a  gas. 

Water  furnishes  a  good  example  of  the  three  states  of 
matter — solid,  liquid,  and  gaseous,  and  how  latent  heat 
affects  these  states. 

Ice  melts  at  32°  and  atmospheric  pressure.  The  mole- 
cules of  the  ice  are  held  together  by  a  certain  force  and  a 
certain  amount  of  heat  is  necessary  to  overcome  this  force 
(142  B.T.U.  per  pound)  and  convert  the  ice  into  water. 
This  heat  is  called  the  latent  heat  of  fusion  and  apparently 


26  GAS-ENGINE  THEORY  AND   DESIGN 

disappears  as  the  temperature  of  the  resulting  water  will 
be  32°,  and  it  is  therefore  insensible  heat  since  it  cannot  be 
perceived  by  the  senses.  Upon  freezing  this  heat  is  again 
restored,  or  liberated.  When  all  the  ice  has  been  melted, 
if  the  heating  is  continued,  the  temperature  of  the  water 
will  rise  until  it  reaches  212°  (180  B.T.U.  are  required  to 
raise  the  temperature  of  one  pound  of  water  from  32°  to 
212°),  when  a  large  amount  of  heat  (966  B.T.U.  per  pound) 
is  necessary  to  convert  the  water  into  steam,  viz.,  a  gas. 
The  heat  which  here  apparently  disappears  is  the  latent 
heat  of  vaporization,  and  the  temperature  of  the  steam  does 
not  rise  above  212°,  at  atmospheric  pressure,  until  all  the 
water  has  been  converted  into  steam.  When  the  steam  is 
condensed  this  latent  heat  is  restored. 

The  latent  heats  vary  with  the  pressure. 

30.  THERMOMETRY. — The  thermometer  is  an  instrument 
for  measuring  differences  in  temperature. 

The  Fahrenheit  (abbreviated  F.)  thermometer  is  gener- 
ally used  in  English-speaking  countries,  while  the  Centi- 
grade (C.)  thermometer  is  used  in  the  countries  which 
have  adopted  the  metric  system.* 

On  the  Fahrenheit  thermometer  the  freezing-point  of 
water  is  taken  at  32°,  and  the  boiling-point  at  212°,  the 
intervening  space  being  divided  into  180  equal  parts,  or 
degrees.  Water  is  taken  as  the  standard  for  the  sake  of 
convenience,  and  at  a  pressure  of  one  atmosphere  at  the 
sea  level. 

On  the  Centigrade  thermometer  the  freezing-point  of 
water  is  taken  at  0°  and  the  boiling-point  at  100°. 

Below  are  given  examples  of  the  method  of  converting 
Fahrenheit  readings  into  Centigrade,  and  vice  versa: 

Q.—  What  temperature  C.  corresponds  to  152°  F.? 

A.— (152— 32)  x  £  =  66f  C. 

*  The  Fahrenheit  scale  is  used  in  this  book. 


HEAT.    THERMODYNAMICS  27 

Q.— What  temperature  F.  corresponds  to  90°  C.? 

A.— (90  X  f )  +  32  =  194°  F. 

The  substances  commonly  used  in  thermometers — mer- 
cury and  alcohol — have  a  limited  range,  freezing  at  low 
and  vaporizing  at  high  temperatures,  so  that  where  a 
greater  range  is  necessary  other  instruments  and  methods 
must  be  employed. 

31.  ABSOLUTE  TEMPERATURE  AND  ABSOLUTE  ZERO. — In 
the  air  thermometer  the  temperature  is  indicated  by  a 
drop  of  mercury  resting  on  a  column  of  air.    If  this  air  is 
exposed  to  atmospheric  pressure  at   the   freezing-point, 
and  the  position  of  the  mercury  marked,  and  then  the  posi- 
tion of  the  mercury  is  marked  at  the  boiling-point  of  water, 
it  will  be  found  that  the  expansion  of  the  original  column 
of  air  amounts  to  36.65  per  cent.    This  gives  an  expansion 
for  each  degree  of  0.3665/180,  or  0.002036.     If  we  now 
assume  that  the  volume  of  air  will  decrease  I/. 002036  for 
each  degree  F.  in  cooling,  i.e.,  in  the  same  ratio,  then  a 
temperature  must  finally  be  reached  where  a  further  re- 
duction in  volume  by  a  reduction  of  temperature  is  im- 
possible.    This  is  called  the  absolute  zero,  and  is  491.13° 
below  the  melting-point  of  ice,  or  459.13°  below  0°  F. 
This  has  been  computed  for  a  perfect  gas  as  492.66°  F.,  or 
273.7°  C.    This  absolute  temperature,  as  will  be  seen  later, 
is  of  great  importance  in  thermodynamic  calculations. 

32.  PYROMETRY. — Pyrometers  are  appliances  for  measur- 
ing, or  observing,  high  temperatures.     One  form  suitable 
for  determining  the  temperature  of  exhaust  gases  contains 
compressed  nitrogen  in  a  tube  above  mercury.     This  is 
.suitable  for  temperatures  up  to  950°  F. 

33.  THERMODYNAMICS.— Thermodynamics  is  the  name 
given  to  the  science  of  heat  energy.    The  two  laws  given 
below  are  used  constantly  in  heat  calculations. 

First  Law. — The  first  law  of  thermodynamics  is  that  heat 


28  GAS-ENGINE  THEORY  AND   DESIGN 

energy  and  mechanical  energy  arc  mutually  convertible 
in  a  definite  ratio,  viz.:  1  heat  unit  (B.T.U.)  equals  778 
foot-pounds. 

Second  Law. — The  second  law  may  be  expressed  alge- 
braically as  follows: 

Q,-Q2      T.-T, 


where  Q,  and  T\  equal  the  quantity  and  absolute  tem- 
perature of  the  heat  received,  and  Q2  and  T2  equal  the 
quantity  and  absolute  temperature  of  the  heat  rejected. 

The  heat  used  in  a  heat  engine  is  the  difference  between 
the  heat  received  and  the  heat  rejected,  and  this,  divided 
by  the  heat  received,  equals  the  thermodynamic  efficiency 
of  the  machine.  The  thermal  efficiency  is  therefore  pro- 
portional to  the  absolute  temperatures  and  expresses  the 
percentage  of  the  heat  used. 


33a.— TABLK   I 

PHYSICAL    PROPERTIES    OF    .MATERIALS 

Solids 


Material 

Specific 

Gravity 

Specific 
Heat 

Weight 
per  Cubic 
Foot 

Coefficient 
of  Linear 
Expansion 

Silver  100 
Thermal 
conduc- 
tivity 

Tempera- 
ture of 
Fusion 
°F. 

Aluminum  

2  .  56 

0.2143 

160      0.0000130 

31.3 

1220 

Brass  

8.32 

0.0939 

520     '0.00001037 

1900 

Bronze  

8.83 

550 

0.00000986 

1700 

Copper  

8.82 

0.0951 

557 

0.00000955 

73.6 

2000 

Iron,  cast  

7.20  !  0.1298 

450 

0.00000617 

11.9 

2192 

Iron,  wrought  .. 

7.7     i  0.1138 

480 

0.00000686 

8.5 

2912 

Lead  

11.37     0.0314 

710 

0.0000162 

8.5 

626 

Steel,  soft  

7.8     i  0.1165 

490 

0.00000599 

11.6 

2520 

Steel,  hard  

.    7.8     ;  0.1175 

490 

0.00000702 

2570 

Tin  

7.29     0.0562 

455 

0.00001230 

15.2 

446 

Zinc  

7.15 

0.0956 

430 

0.00001634 

28.1 

786 

HEAT   THERMODYNAMICS 

Liquids 


29 


Specific 
Gravity 

Specific 
Heat 

Weight 
per  Cubic 
Foot 

Tempera- 
ture 
Fusion 

Tempera- 
ture Vapor- 
ization 

Latent 
Heat  Va- 
porization 

Water.  .  . 
Alcohol  

1.0000 
0.794 

1.0000 
0.6200 

62.4 
49.6 

32° 
-202.9 

212° 
173 

966  B.T.U. 

372 

336.  Density  =  specific  weight  =  weight  of  a  unit  volume. 
Vapor  is  a  gas  below  the  critical  temperature,  i.e.,  it  can 
be  reduced  to  a  liquid  by  pressure  alone. 


CHAPTER  V 

COMBUSTION 

34.  CHEMISTRY  OF  COMBUSTION'. — An  elementary  knowl- 
edge of  chemistry  is  necessary  for  the  understanding  of  the 
process  of  combustion. 

An  element  is  a  substance  that  cannot  be  separated  into 
anything  else.  Example:  Iron,  carbon,  oxygen. 

A  compound  is  a  substance  that  can  be  separated  into 
elements.  Example:  Y\'ater  can  be  separated  into  oxygen 
and  hydrogen. 

An  atom  is  the  smallest  particle  of  matter  that  can  exist. 

A  molecule  is  the  smallest  quantity  into  which  a  mass  of 
matter  can  be  divided  without  changing  its  chemical  nature. 
Every  molecule  consists  of  two  or  more  atoms. 

A  mechanical  mixture  is  one  in  which  substances  are  not 
chemically  combined.  Example:  Salt  water. 

A  physical  change  is  one  in  which  the  nature  of  the  sub- 
stance is  not  changed.  Example:  Water  converted  into 
steam. 

A  chemical  change  is  one  in  which  the  nature  of  the  sub- 
stance is  changed.  Example :  The  burning  of  a  piece  of  coal. 

Atomic  weight  is  the  weight  of  an  atom  of  any  element  as 
compared  with  hydrogen,  the  lightest  known  element. 

Chemical  nomenclature. — Abbreviations  are  used  in  chem- 
istry for  the  names  of  the  various  elements,  in  writing  the 
reactions.  Some  of  these  are  given  in  the  following  para- 
graphs. 

Chemical  action   is   most   energetic    between   dissimilar 


COMBUSTION  31 

substances  and  takes  place  under  certain  conditions  only. 
It  is  very  difficult  to  bring  about  chemical  action  by 
mechanical  means,  as  it  is  effective  at  insensible  distances 
only,  and  for  this  reason  the  agents  usually  employed  are 
solvents  and  heat.  Chemical  combination  always  takes 
place  in  certain  definite  proportions;  for  example,  when 
hydrogen  and  oxygen  are  brought  together  and  chemical 
action  is  started  by  heating,  2  atoms  of  hydrogen  will  unite 
with  1  atom  of  oxygen  (written  2H+0  =  H,O)  forming 
water,  and  the  elements  will  not  combine  with  each  other 
in  any  other  proportion. 

35.  COMBUSTION  may  be  defined  as  the  chemical  com- 
bination of  one  or  more  elements  with  oxygen,  taking  place 
with  sufficient  rapidity  to  be  accompanied  by  heat  and 
light, 

Let  us  examine  this  matter  of  chemical  action  and  com- 
bustion more  closely.  In  order  to  burn  a  substance — let 
us  take  a  piece  of  coal  for  example — it  must  be  heated  to 
a  certain  temperature  before  it  will  ignite.  This  temperature 
of  ignition  varies  with  different  substances.  When  the 
coal  is  first  heated  gases  are  formed  on  the  outside  (where 
the  heat  acts  first)  and  these  gases,  combining  with  the 
oxygen  of  the  air,  undergo  the  chemical  transformation 
called  "combustion."  The  heat  now  given  off  by  the  part 
of  the  coal  that  is  burning  will  heat  the  rest  of  the  coal  to 
the  temperature  required  for  ignition,  and  thus  the  com- 
bustion goes  on  until  all  the  coal  has  been  consumed.  In 
place  of  a  piece  of  coal  we  now  have  a  certain  bulk  of  very 
hot  gas  and  a  little  ash.  In  burning  nothing  is  destroyed, 
combustion  is  simply  a  chemical  change  in  which  the 
burning  elements  enter  into  new  combinations  which  we 
now  have  in  the  form  of  hot  gases.  These  gases  and  the 
ash  weigh  just  as  much  as  the  coal  and  air  consumed  did, 
nothing  has  been  lost,  but  the  heat  stored  in  the  coal  has 


32  GAS-ENGINE  THEORY   AND   DESIGN 

boon  liberated.  Air  is  a  mechanical  mixture  of  oxygen  and 
nitrogen,  the  proportion  being  about  one  part  of  the  former 
to  three  parts  of  the  latter  by  weight.  Oxygen,  as  men- 
tioned before,  possesses  the  property  of  entering  into 
chemical  combination  with  many  substances  after  they 
have  been  heated  to  a  sufficiently  high  temperature,  and 
it  is  for  this  reason  that  air  is  necessary  for  combustion 
since  it  furnishes  the  required  oxygen.  This  changing  of 
a  fuel,  first  into  gas  by  raising  its  temperature,  and  then 
burning  the  gas,  takes  place  no  matter  whether  the  fuel 
is  a  solid  or  a  liquid. 

When  carbon  is  burned  completely  it  burns  to  a  gas 
called  carbon  dioxide.  Each  atom  of  carbon  combines  with 
two  atoms  of  oxygen,  and  the  reaction  is  written  as  follows: 

C  +  20  =  C02 

If  the  carbon  is  in  combination  with  another  element, 
the  preliminary  heating  weakens  the  force  which  holds  it 
in  combination  and  the  C  and  O  pull  together  as  a  magnet 
and  a  piece  of  iron  pull  together.  Because  heating  weakens 
the  force  holding  the  elements  together  it  is  necessary  to 
heat  most  fuels  before  they  will  burn.  To  separate  the 
C  and  0  after  combustion  requires  just  as  much  heat  as 
was  liberated  during  the  combustion.  Since  the  sun's  heat 
in  the  first  place  brought  about  the  chemical  changes  by 
which  fuels  were  formed,  the  sun  is  the  source  of  the  energy 
stored  in  the  fuels. 

36.  A  FLAME  is  a  current  of  hot  gas  carrying  with  it 
solid  particles  at  such  a  temperature  as  to  glow  and  give 
out  heat  and  light. 

37.  IGNITION  is  the  first  step  in  combustion,  i.e.,  it  is 
the  beginning  of  the  chemical  combination. 

38.  SMOKE  is  a  current  of  burnt  gases  carrying  with  it 
particles  of  unburnt  carbon. 


COMBUSTION  33 

39.  An  EXPLOSION  is  extremely  rapid  combustion. 

40.  SPONTANEOUS  COMBUSTION  occurs  when  a  body  ab- 
sorbs oxygen  so  rapidly  that  the  chemical  combination 
raises  the  temperature  sufficiently  so  that  it  will  burst  into 
a  flame. 

41.  COMPLETE  COMBUSTION  of  a  fuel  element  is  its  com- 
bination with  that  amount  of  oxygen  which  produces  the 
most  stable  compound.     Example:    C  burning  to  C02. 
During  complete  combustion  no  flame  is  visible. 

42.  INCOMPLETE  COMBUSTION  of  a  fuel  element  is  its 
combination  with  oxygen  in  such  proportions  as  to  form 
an  unstable  compound.    Example:  C  burning  to  CO. 

43.  A  FUEL  is  a  substance  containing  elements  which 
will  combine  with  oxygen  under  proper  conditions  and  in 
so  doing  produce  heat.    A  fuel  may  be  a  solid,  a  liquid, 
or  a  gas. 

44.  CALORIFIC  POWER. — The  complete  combustion  of  a 
unit  weight  of  any  fuel  element  produces  a  definite  quan- 
tity of  heat.    This  is  called  its  calorific  power  and  is  ex- 
pressed in  heat  units. 

45.  ASH  is  incombustible  matter  contained  in  a  fuel. 

46.  SPEED  OF  COMBUSTION. — This  depends  upon  a  num- 
ber of  conditions  and  therefore  varies  greatly.    It  depends 
upon: 

(a)  The  temperature  before  ignition,  increasing  with  this 
temperature; 

(6)  The  elements  of  which  the  fuel  is  composed,  some 
elements  burning  faster  than  others; 

(c)  Proportion  of  diluents, etc., decreasing  as  an  excess  of 
air  is  provided; 

(d)  The  more  intimately  the  gases  and  oxygen  are  mixed 
the  quicker  the  combustion ; 

(e)  The  greater  the  compression  the  quicker  the  com- 
bustion. 

3 


34  GAS-ENGINE  THEORY   AND   DESIGN 

47.  COMPOSITION  OF  Am. — Air  contains  oxygen  ami  nitro- 
gen in  the  following  proportions  at  32°,  and  at  atmospheric 
pressure: 

By  Weight.         By  Volume. 

Oxygen. .. .0.236  0.213 

Nitrogen.  ..0.764  0.787 

48.  AIR  REQUIRED  FOR  THE  COMBUSTION  OF  CARBON  TO 
CARBON  DIOXIDE. — 

C  +  20  =  C02 
12  +  32  =44 

12  is  the  atomic  weight  of  C,  and  16  is  the  atomic  weight 
of  O.  For  burning  1  Ib.  of  C,  32/12  Ibs.  of  0  are  required, 
or  2.66  Ibs.  The  amount  of  air  required  will  be  32/12x 
100/23  or  11.6  Ibs.  The  products  of  combustion  are: 

N 8.93  Ibs. 

C02 3.66    " 

Total 12.6  Ibs. 

Nitrogen  is  inert  so  far  as  combustion  is  concerned. 
At  a  temperature  of  32°,  and  a  pressure  of  1  atmosphere, 
1  Ib.  of  air  occupies  12.39  cu.ft. 
The  total  amount  of  air  required  in  cu.ft.  is: 

12.39  X  11.6  =  143,  or  about  153  cu.  ft.  at  62°. 

49.  AIR  REQUIRED  FOR  THE  COMBUSTION  OF  CARBON 
MONOXIDE  TO  CARBON  DIOXIDE. — 

CO  +  0=C02 
28  +  16=44 

The  weight  of  air  required  will  be : 

16/28  X  100/23:  2. 48  Ibs. 
The  air  required  in  cu.  ft.  will  be: 

12. 39X2. 48  =  30. 75  at  32°. 


COMBUSTION  36 


The  products  of  combustion  are: 

N 1.91  Ibs. 

C02 1.57    " 


Total 3.48  Ibs. 

50.  Am  REQUIRED  FOR  THE  COMBUSTION  OF  HYDROGEN 
TO  H20.— 

2H  +  0  =  H20 
2    +16  =  18 

The  weight  of  air  required  will  be: 

100/23X8  =  34. 8  Ibs. 
The  air  required  in  cu.  ft.  will  be: 

12.39  X  34.8=431.52 
The  products  of  combustion  are: 

N 26.8  Ibs. 

H2O . .  9 


Total 35.8  Ibs. 

The  H20  is  in  the  form  of  superheated  steam. 

51.  AIR  REQUIRED  FOR  THE  COMBUSTION  OF  SULPHUR 
TO  SULPHUR  DIOXIDE. — 

S  +  20  =  S02 
32+32  =64 

The  weight  of  air  required  will  be: 

100/23  X  1=4. 34  Ibs. 
The  air  required  in  cu.  ft.  will  be: 

12.39  X  4. 34  =  53 
The  products  of  combustion  are: 

N.. 3.35  Ibs. 

S02 2 

Total 5.35  Ibs. 


36  GAS-ENGINE  THEORY   AND   DESIGN 

52.  COMBUSTION  OF  A  COMPOUND.— Experiment  has 
shown  that: 

When  a  fuel  contains  0,  then  so  mueh  less  0  will  be  re- 
quired for  complete  combustion; 

When  a  fuel  contains  both  H  and  O,  then  only  the  surplus 
H  (if  any)  need  be  considered.  The  H  and  0  of  the  fuel 
used,  in  forming  H2O,  will  not  affect  the  calorific  power 
of  the  fuel  and  may  be  neglected.  This  gives  a  high  ".ml 
a  low  heating  value  for  such  a  fuel,  the  low  value  being  the 
true  one. 

Example:  A  sample  of  anthracite  coal  has  been  analyzed 
and  found  to  be  as  follows: 

C 94  per  cent. 

H  and  O  in  proportions  to  form  water.  .  .     4    " 

II  available  for  combustion 2    "       " 

The  cu.  ft.  of  air  required  for  the  combustion  of  C  will  be 
143  X  94 

100 
and  for  H 

431  X2 


100 

The  analysis  gives  percentages  by  weight.  The  cu.  ft. 
of  air  required  can  be  found  direct  without  first  finding 
the  weight  of  air,  but  the  student  should  not  confuse 
weights  and  volumes. 

A  formula  often  used  for  finding  the  weight  of  air  re- 
quired is: 

(1)  Weight  of  air  in  Ibs.  =  12  C  +  36(  -£) 

52a.  THE  CALORIFIC  POWER  OF  A  COMPOUND  can  be 
figured  according  to  the  percentage  of  the  elements.  The 
H  and  O  of  the  fuel  must  be  treated  as  described  in  the 
preceding  paragraph. 


COMBUSTION  37 

Note. — It  must  be  understood  that  C  and  0  may  exist 
in  a  fuel  without  being  in  the  form  of  CO,  or  C02,  and  that 
H  and  S  may  also  be  in  a  fuel  without  being  combined 
with  0. 

More  air  is  always  supplied  in  gas  engines  than  is  theoret- 
ically required  for  combustion. 

53.  VOLUME  OF  THE  PRODUCTS  OF  COMBUSTION. — It  will 
be  shown  later  that  V°T  =  VT°. 

Where  V°  is  the  volume  of  the  gas  at  the  freezing-point. 
T°    "  temperature  abs. 

V     "  actual  volume. 

T      "  temperature. 

This  is  true  only  when  no  chemical  reactions  are  going 
on  during  expansion.  The  combined  volume  after  com- 
bustion may  not  be  the  same  as  the  sum  of  the  volumes 
before  combustion,  even  when  the  final  and  initial  tem- 
peratures and  pressures  are  the  same.  For  example: 

1  cu.  ft.  of  H  requires  2.37  cu.  ft,  of  air,  total  3.37  cu.  ft. 
The  volume  after  combustion  will  be  2.87  cu.  ft. 

1  cu.  ft.  of  CO  requires  2.37  cu.  ft.  of  air,  total  3.37  cu.  ft. 
The  volume  after  combustion  will  be  2.87  cu.  ft. 

1  cu.  ft.  of  CH4  requires  9.50  cu.  ft.  of  air,  total  10.50  cu. 
ft.  The  volume  after  combustion  will  be  10.50  cu.  ft. 

The  Law  of  Avogadro  must  here  be  taken  into  account. 
This  law  is  as  follows: 

Equal  volumes  of  all  gases  under  the  same  conditions  of 
temperature  and  pressure  contain  the  same  number  of 
molecules. 

(2)  Weight  in  Ibs.  per  cu.  ft.  at  32°  - 

Molecular  weight  X  0.00559 

2 

Following  are  the  volumes  in  cu.  ft.  per  Ib.  of  some  gases 
at  62°: 


38  GAS-ENGINE  THEORY   AND   DESIGN 

CO, 8.6   cu.  ft. 

H." 190 

S02 5.85     " 

N 13.5       " 

54.  THEORETICAL  TEMPERATURES  OF  COMBUSTION. — 

a.  When  C  burns  to  C02 

Number  of  B.  T.  U.  available  =  14,500  per  Ib.  of  C. 
For  every  degree  rise  in  temperature  N  requires 
8.93  X 0.244  B.  T.  U.  And  CO,  requires  3.66  X 
0.216  B.  T.  U.  (multiplying  the  weight  in  Ibs.  by  the 
specific  heat)  then  t  [(8.93x0.244) +  (3.66x0.216)] 
=  14,500  t=  4872°  =  increase  in  temperature. 

If  we  assume  the  temperature  of  the  air  before  com- 
bustion to  have  been  62°,  then  the  final  temperature  will 
be  4872 +62  =4934°. 

b.  When  CO  burns  to  CO., 

Number  of  B.T.  U.  available  =4,320  per  Ib.  of  CO 

4320 

=  (1.9  X  0.244)  + (1.57  X0.216r538°°    ™    '" 
temperature. 

c.  W lien  H  burns  to  H20 

Here  the  temperature  of  the  water  must  first  be 
raised  to  the  boiling-point.  Latent  heat  must  be 
supplied  to  convert  the  water  into  steam.  The 
steam  must  be  raised  to  the  final  temperature. 
The  N  must  be  raised  to  the  final  temperature. 

Number  of  B.  T.  U.  available  =  62,000  per  Ib.  of  H. 
62,000 -(9X966)  =53,306 
(212-t.)  9+  (t-212)(9x0.48)  +  (t-t1)  (26.8  X 

0.244)  =  53,306 
t=4910° 
t,  =  65°= temperature  of  air  before  combustion. 


COMBUSTION  39 

The  theoretical  temperatures  are  never  attained  because: 
the  combustion  is  seldom  complete;  an  excess  of  air  is 
always  supplied;  there  are  radiation  losses;  some  moisture 
is  usually  present  in  the  fuel;  dissociation  takes  place. 

55.  DISSOCIATION. — The  tendency  of  carbon  to  combine 
with  oxygen  increases  with  the  temperature,  as  has  been 
stated  before,  until' it  reaches  a  certain  limit,  and  it  then 
decreases  with  a  further  increase  in  temperature,  the 
affinity  between  the  elements  finally  becoming  zero,  and  a 
still  further  increase  in  temperature  results  in  a  separation 
of  the  CO  or  C02,  or  other  combinations  with  0.  This 
breaking  up  of  the  chemical  combination  with  0  is  called 
"dissociation,"  and  takes  place  with  a  corresponding  ab- 
sorption of  heat,  lowering  the  temperature  of  the  fire  and 
decreasing  the  calorific  power  of  the  fuel,  since  it  takes 
just  as  much  heat  to  break  up  an  oxygen  compound  as  was 
liberated  in  the  formation  of  that  compound.  The  H20 
present  is  also  broken  up.  When  the  temperature  has  fallen 
sufficiently  the  oxygen  compounds  again  form  and  re- 
supply  the  lost  heat,  but  in  a  gas  engine  this  re-combination, 
or  "after- burning,"  may  occur  so  late  in  the  stroke  as  to 
constitute  a  heat  loss.  Dissociation  is  supposed  to  take 
place  at  temperatures  ranging  from  2,500°  up.  The  tem- 
perature varies,  of  course,  with  conditions  and  cannot  be 
accurately  determined. 


CHAPTER  VI 

FUELS 

56.  The  fuels  commonly  used  in  cdnnection  with  heat 
engines  are:  solid,  anthracite  and  bituminous  coal;  liquid, 
petroleum  and  its  distillates,  alcohol ;  gas,  the  various  gases 
made  from  coal,  natural  gas;  other  fuels  are  coke,  charcoal, 
wood,  peat,  etc.,  but  since  they  are  used  only  to  a  limited 
extent  they  will  not  be  further  considered  here. 

57.  COAL. — Regarding  the  origin  of  coal,  geology  teaches 
us  that  many  ages  ago  the  earth  was  very  different  from 
what  it  is  to-day.     Continents  slowly  emerged  from  seas 
and  great  changes  on  land  and  water  took  place.     During 
one  era,  called  the  carboniferous  era,  the  air  was  very  hot 
and  moist  and  the  land  was  covered  with  immense  tropical 
forests  and  other  luxuriant  vegetation.    During  succeeding 
ages  further  changes  took  place  and  these  forests  were  even- 
tually buried  beneath  deposits  of  sand,  rock,  etc.,  until  a 
great  crust  had  been  formed  over  them.    Decomposition  of 
the  vegetable  matter  produced  heat  and  the  heat  and 
pressure  resulted  in  a  partial  combustion  and  distillation 
of  the  vegetable  products,  changing  them  into  coal.    This 
coal,  as  we  all  know,  is  now  obtained  by  mining.    Coal  is 
generally  classified  as  anthracite,  or  hard,  and  bituminous, 
or  soft.    In  this  country  Pennsylvania  is  the  great  anthra- 
cite-producing State.    The  output  for  1905  was  78,731,523 
tons.    About  28  States  produce  bituminous  coal,  and  the 
total  output  for  the  same  year  was  310,040,644  tons.    The 
production  of  coal  in  the  United  States  during  the  calendar 
year  1907  was  428,895,914  tons  of  2,240  Ibs.  each.    Great 

40 


FUELS  41 

Britain  and  Germany  rank  next  as  coal-producing  coun- 
tries. The  chief  reason  why  greater  progress  has  been  made 
in  Europe  with  large  gas  engines  than  here  is  because  fuel 
is  so  much  scarcer  there  and  the  natural  resources,  in 
general,  are  more  limited,  necessitating  greater  economy. 
Since  the  composition  of  coal  varies  much  the  calorific  power 
varies  accordingly.  In  order  to  obtain  a  fair  average  of 
the  heating  value  of  coal  from  a  certain  locality  several 
samples  should  be  analyzed. 

58.  Anthracite  is  a  hard  coal,  burning  with  little  or  no 
smoke.    Its  calorific  power  is  usually  greater  than  that  of 
soft  coal.    It  contains  a  certain  amount  of  incombustible 
matter  commonly  termed  "ash."    An  average  composition 
is  as  follows: 

Carbon 90  per  cent  by  weight. 

H,  orOandN 5  "       " 

Water 1   "       " 

Ash 4   "      "  " 

B.  T.  U.  per  pound  13,000  to  14,000. 

59.  Bituminous,  or  soft  coal,  ignites  more  readily  than 
the  anthracite,  and  during  combustion   (under  a  boiler) 
usually  gives  off  a  large  amount  of  black  smoke.    A  "good 
average"  analysis  of  bituminous  coal  cannot  be  given  since 
it  varies  from  grades  rich  in  heating  value  to  grades  almost 
unfit  for  use  as  a  fuel.    The  following,  however,  may  be 
taken  as  a  guide: 

Carbon 50-80  per  cent  by  weight. 

H,  0,  N,  etc 10-40  "      " 

Sulphur 1-  3   "       " 

Ash,  or  earthy  matter,  2-20   "       "  " 

B.T.U.  per  pound  9,000  to  14,000. 

There  are,  of  course,  other  kinds  of  coal  such  as  semi- 
bituminous,  lignite,  etc.,  but  these  need  not  be  discussed  here. 


42 


GAS-ENGINE   THEORY  AND   DESIGN 


60.  PETROLEUM. — Petroleum  is  also  called  mineral  and 
crude  oil.  With  respect  to  its  origin  opinions  differ,  but  it 
seems  that  this  oil  is  the  result  of  the  decomposition  of 
animal  matter  which  was  buried  in  a  similar  manner  to 
the  coal.  The  United  States  is  the  greatest  oil-producer 
in  the  world,  with  Russia  second.  The  oil  is  obtained  by 
boring  wells  in  the  oil  districts.  Sometimes  the  petroleum 
has  to  be  pumped  out,  but  frequently  it  gushes  out  of  the 
well  with  considerable  force.  Petroleum  is  found  in  this 
country  in  Pennsylvania,  Texas,  California,  and  some  other 
States.  The  annual  production  in  the  United  States  in 
1905  reached  a  total  of  117,090,772  barrels  of  42  gallons 
each.  During  the  calendar  year  1907  the  production  of 
petroleum  was  6,976,004,070  gallons.  In  order  to  obtain 
the  various  oils  known  as  gasolene,  kerosene,  etc.,  the  crude 
oil  is  subjected  to  a  distillation  process.  The  lighter  oils 
are  driven  off  first  and  the  result  of  the  distillation,  and 
general  properties  of  the  distillates,  are  given  in  Table  II. 


60a.— TABLE  II 

PETROLEUM   DISTILLATES 


Tempera- 
ture 

Distillate 

Per  Cent 

Specific 
Gravity 

Flashing 
Point 

B.  T.  U. 

per  Ib. 

Baume 
Degree  at 
60°  F. 

113 
113-140 
140-160 
160-250 
250-350 
340 
480 

Rhigolene  
Chymogene  
Gasolene  
Benzine  
Naphtha  
Kerosene  
Lub.  oil  
Paraffin  
Residue  

Traces 
Traces 
1.5 
2-10 
2-10 
50 
15 
2 
16 

.59-.  62 
.59-.  62 
.65-.  72 
.74 
.74 
.98 
.90 

40^70 
14-32 
14-32 
100-160 
230 

18,000 
18,000 
18,000 
22,000 

"QS 

48 

Distillate  
Crude  oil  

.80-.  90 

.88 

17,000-20,000 
19,000-22,000 

22 
30 

FUELS  43 

The  specific  gravity  of  a  substance  is  the  weight  of  a  unit 
volume  as  compared  with  the  weight  of  a  unit  volume  of 
water. 

The  flashing-point  of  a  substance  is  the  temperature  at 
which  it  gives  off  an  ignitible  vapor. 

Fractional  distillation  is  the  separation  of  different  con- 
stituents from  a  substance.  An  example  of  this  is  the  dis- 
tillation of  petroleum. 

Destructive  distillation  is  the  heating  of  a  substance  from 
which  air  has  been  excluded.  This  is  employed  in  making 
coke  and  charcoal. 

Hydrocarbons  is  the  name  given  to  the  various  combina- 
tions of  hydrogen  and  carbon.  To  this  class  belong  the 
petroleum  products,  alcohol  and  natural  gases. 

Petroleum  has  an  average  composition  of:* 

Carbon 85  per  cent. 

Hydrogen 13   "       " 

0  and  impurities 2   "      " 

Crude  petroleum  gives  off  a  disagreeable  odor  and  its 
volatile  constituents  make  it  dangerous  in  confined  spaces. 
On  account  of  its  impurities,  and  the  difficulty  of  obtaining 
complete  combustion,  it  is  difficult  to  use  in  the  engine 
cylinder  direct  except  with  high  compression — as  in  the 
case  of  the  Diesel  engine.  The  gummy  residue  resulting 
from  incomplete  combustion  would  soon  cause  trouble  in 
the  engine  cylinder.  Engines  are  sometimes  operated  on 
gas  made  from  the  crude  oil  (see  Oil  Water  Gas).  The  great 
advantage  of  crude  oil  is  its  cheapness. 

61.  Fuel  oil,  or  fuel  distillate,  is  the  product  left  over 
when  distillation  is  stopped  after  the  kerosene  has  been 
obtained.  It  is  safer  than  the  crude  oil  since  the  more 
volatile  constituent  shave  been  driven  off.  The  problems  of 
carburation  (where  a  carbureter  is  used)  and  of  ignition, 


44  GAS-ENGINE  THEORY  AND   DESIGN 

however,  are  difficult  ones.  Unless  complete  combustion 
is  obtained  the  exhaust  will  be  smoky  and  give  off  disagree- 
able odors.  There  is  also  danger  from  carbon  deposits  in 
the  engine  cylinder.  If  practically  perfect  combustion  can 
be  obtained  it  forms  a  safe  and  cheap  fuel. 

There  are  so  many  grades  of  fuel  oil  that,  when  the  oil 
is  wanted  for  a  gas  engine,  this  fact  should  be  stated  in 
ordering,  since  some  fuel  oils,  as  well  as  some  crude  oils, 
are  absolutely  unfit  for  use  in  the  engine  cylinder. 

Since  the  fuel  oil  and  distillates  contain  various  com- 
binations of  hydrogen  and  carbon,  it  is  difficult  to  give  the 
chemical  composition. 

62.  Kerosene  is  a  cheap  and  safe  fuel  and  can  be  used  to 
good  advantage  in  the  gas  engine  by  increasing  the  com- 
pression somewhat  over  that  commonly  used  for  gasolene, 
and  by  properly  carbureting.    Its  use  as  a  fuel  for  small 
motors  is  spreading  rapidly.    While  the  probable  percent- 
age of  kerosene  in  petroleum  is  about  50,  only  30  per  cent 
is  actually  obtained  by  distillation. 

One  pound  of  kerosene  vapor,  at  atmospheric  pressure, 
occupies  a  space  of  2.47  cu.  ft. ;  188  cu.  ft.  of  air  are  required 
for  its  combustion,  or  volumes  in  a  ratio  of  1  to  76 ;  it  is 
about  five  times  as  heavy  as  air. 

A  number  of  the  so-called  "kerosene"  engines  are  first 
started  on  gasolene  and  when  the  engine  is  warmed  up 
the  gasolene  is  turned  off  and  the  kerosene  on. 

63.  Gasolene   has   been   used   for   automobile   and   the 
smaller  marine  and  stationary  engines,  almost  exclusively 
because  of  the  following  advantages:   It  is  easily  carbur- 
eted, since  it  is  volatile ;  it  is  readily  ignited  by  the  electric 
spark;   good  combustion  is  more  easily  obtained  than  in 
the  case  of  the  heavier  oils;  in  short,  it  is  cleaner  and  easier 
to  handle. 

It  possesses  a  number  of  disadvantages,  however,  among 


FUELS  45 

which  are  the  following:  It  is  volatile,  therefore  dangerous 
unless  carefully  handled;  more  expensive  than  the  other 
oils;  in  some  localities  the  cost  is  almost  prohibitive; 
considerable  loss  from  evaporation  occurs  in  warm  locali- 
ties; cannot  be  procured  at  all  in  some  places;  in  some 
of  the  South  American  countries  its  use  is  forbidden 
by  law. 

One  pound  of  gasolene  vapor,  at  atmospheric  pressure, 
occupies  a  space  of  4.06  cu.  ft. ;  181  cu.  ft.  of  air  are  required 
for  its  combustion,  or  volumes  in  a  ratio  of  46.6  to  1 ;  it  is 
about  3.05  times  as  heavy  as  air. 

There  are  several  grades  of  gasolene  and  this  fact  must 
be  considered  in  specifying  it  for  fuel  purposes. 

64.  ALCOHOL. — There  are  two  kinds  of  alcohol — methylic, 
or  wood  alcohol,  C2H4O2,  made  by  dry  distillation  of  wood 
in  iron  retorts,  and  ethyl  alcohol,  C2H602,  made  by  dis- 
tillation from  the  fermented  infusions  of  substances  con- 
taining starch,  such  as  potatoes,  corn,  rice,  barley,  wheat, 
etc.,  or  substances  containing  sugar,  such  as  sugar  beets, 
sugar  cane,  molasses,  etc.  Waste  products  can  be  used, 
such  as  diseased  potatoes,  bitter  molasses,  etc. 

For  fuel  purposes  the  ethyl  alcohol  is  denaturized  so  as 
to  make  it  unfit  for  human  consumption.  This  is  done  by 
adding  benzine,  wood  alcohol,  gasolene,  and  other  sub- 
stances. 

There  are  also  electrical  methods  of  manufacturing  alco- 
hol, and  for  these  a  great  deal  is  claimed. 

Following  are  the  principal  properties  of  ethyl  alcohol: 
Specific  gravity,  0.79;  freezing-point,  about  200°  below 
zero  when  pure;  calorific  power,  28,000  B.T.U.  per  Ib. 
when  pure;  when  diluted  this  may  run  down  to  12,000 
B.T.U. ;  has  a  strong  affinity  for  water. 

A  somewhat  higher  compression  than  for  gasolene  is 
usually  required  on  account  of  the  water  in  the  alcohol. 


46  GAS-ENGINE  THEORY   AND   DESIGN 

Until  quite  recently  the  extensive  manufacture  of  alcohol 
was  prevented  by  an  excessive  tax,  but  this  has  been  re- 
moved, so  that  alcohol  can  now  be  manufactured  in  quantity, 
although  under  certain  restrictions.  When  produced  in 
large  quantities  it  can  be  sold  at  a  price  that  compares 
favorably  with  that  of  gasolene  or  kerosene.  The  Treasury 
Department  gives  the  following  figures  for  denatured  al- 
cohol: For  the  six  months  ending  June  30th,  1907,  whole- 
sale dealers  received  1,724,062  wine  gallons  (this  includes 
alcohol  received  from  other  dealers)  and  sold  and  removed 
1,441,360  gallons. 

Among  its  principal  advantages  as  a  gas-engine  fuel  are: 
It  can  be  produced  in  any  quantity  and  at  a  comparatively 
low  cost;  it  is  safe,  fires  can  be  extinguished  with  water; 
this  cannot  be  done  in  the  case  of  oil,  where  water  simply 
spreads  the  flames;  it  is  cleaner  than  the  oils  and  leaves 
no  deposits  in  the  engine  cylinder;  there  is  no  danger  from 
explosions  in  case  of  leaky  connections. 

The  consumption  in  pounds  per  H.-P.  hour,  however, 
is  greater  than  for  the  fuel  oils  when  the  alcohol  is  greatly 
diluted. 

Under  certain  conditions  acids  are  formed  during  com- 
bustion which  will  corrode  metallic  surfaces. 

Alcohol  is  an  ideal  fuel  for  small  engines  properly  de- 
signed for  its  use. 

65.  NATURAL  GAS  is,  or  was,  formed  by  underground  dis- 
tillation.    It  is  obtained,  as  in  the  case  of  petroleum,  by 
drilling  to  the  subterranean  accumulations.    Where  obtain- 
able it  forms  an  ideal  fuel,  especially  for  the  larger  station- 
ary gas  engines.     The  calorific  power  can  readily  be  cal- 
culated when  the  chemical  composition  is  known. 

66.  COAL  GAS,  or  ILLUMINATING  GAS,  is  obtained  by  de- 
structive distillation  of  coal.    The  coal  is  placed  in  closed 
iron  retorts  which  are  heated  from  the  outside.    The  gases 


FUELS  47 

which  are  driven  off  during  this  heating  are  filtered 
and  cooled.  The  product  remaining  in  the  retorts  is 
called  coke.  This  gas  has  been  largely  superseded  by 
water  gas. 

67.  WATER  GAS  is  formed  by  blowing  air  and  steam  alter- 
nately through  a  mass  of  incandescent  carbon.    It  is  similar 
to  the  product  called  "producer  gas."     For  illuminating 
purposes  the  water  gas  is  enriched  by  the  addition  of  car- 
buretted  hydrocarbon  vapors.     The  illuminating  gas  is 
"fixed"  (made  a  stable  compound)  the  same  as  coal  gas, 
by  passing  it  through  a  superheater.    An  illuminating  gas 
is  not  so  good  for  power  purposes  as  a  power  gas. 

68.  OIL  WATER  GAS  is  sometimes  made  from  petroleum 
by  heating  the  oil  in  a  retort  into  which  highly  superheated 
steam  is  passed  in  such  a  manner  as  to  ultimately  mix  the 
constituents.    While  this  gas  has  been  used  as  an  engine 
fuel,  it  is  more  of  an  illuminating  than  a  power  gas. 

69.  BLAST-FURNACE  GAS. — In  the  modern  blast  furnace 
for  the  reduction  of  iron  ore  less  than  one-third  of  the  car- 
bon burns  to   C02,  so  that  the  discharged   gas  consists 
largely  of  CO.    The  CO  has  a  calorific  power  of  about  100 
B.T.U.  per  cu.  ft.,  or  about  1,280  B.T.U.  per  pound.    By  in- 
creasing the  compression  sufficiently  this  gas  can  be  used 
in  the  gas  engine  (see  Par.  144).    About  0.80  cu.  ft.  of  air 
is  required  per  cu.ft.  of  gas,  and  the  calorific  power  of  the 
mixture  is  about  oo  B.T.U.  per  cu.ft. 

The  following  figures  were  furnished  by  the  Lackawanna 
Steel  Company,  and  represent  the  average  blast-furnace 
practice  per  ton  of  iron: 

Charge  Production 

Ore...  .  3,600 Ibs.  Iron 2,240 Ibs. 

Coke 2,000   "  Gas 10,600   " 

Limestone 1,200"  Slag 1,210" 

Air 7,250   " 


14, 050  Ibs.  14,050  Ibs. 


48  GAS-ENGINE  THEORY  AND   DESIGN 

Average  analysis  of  gas  by  weight: 

Per  cent 
by  Weight 

Nitrogen,  N 52.18 

Carbon  monoxide,  CO 26.83 

Carbon  dioxide,  CO2 18 . 23 

Methane,  CH4 38 

Hydrogen,  H 08 

Water  vapor,  HSO...  .     2.30 

Total 100.00 

The  calorific  power  of  the  above  gas  equals  1,283  B.T.U. 
per  Ib.  About  50  per  cent  of  this  is  available  as  a  fuel  for 
gas  engines,  or  5,300  Ibs.  of  gas,  containing  1,283  B.T.U. 
per  Ib.,  for  every  ton  of  pig  iron  produced. 

At  one  time  these  gases  were  considered  waste  products 
and  discharged  directly  into  the  atmosphere. 

One  of  the  chief  problems  in  connection  with  the  use  of 
blast-furnace,  as  well  as  coke-oven  and  producer  gas,  is 
the  proper  cleaning  of  the  gas  before  it  reaches  the  engine 
cylinder.  A  centrifugal  cleaner  for  blast-furnace  gas  con- 
sists of  a  drum  which  revolves  rapidly  inside  of  a  casing 
and  is  so  arranged  that  it  throws  both  water  and  the  gas 
against  the  inside  of  the  casing  by  centrifugal  force.  The 
water  picks  up  the  dust  in  the  gas  and  as  it  drains  off  at 
the  bottom  of  the  casing  it  carries  the  impurities  with  it. 
The  cooled  and  cleaned  gas  passes  on  to  the  engine. 

70.  PRODUCERS  AND  PRODUCER  GAS. — Producers  are  of 
two  kinds:  pressure  producers,  used  for  large  power  in- 
stallations, and  suction  producers,  used  for  the  smaller 
installations. 

In  the  pressure  producer  compressed  air  is  introduced  into 
the  ash-pit  and  the  pressure  throughout  the  system  is 
greater  than  atmospheric.  This  necessitates  an  auxiliary 
air-compressing  system,  a  gas-holder,  and  is  also  open  to 
the  objection  that  if  there  should  be  a  leak  the  CO  will 
escape.  This  gas  is  a  deadly  poison.  The  use  of  a  gas- 


FUELS 


49 


holder  has  several  advantages:  the  action  of  the  producer 
is  not  affected  by  the  pulsations  of  the  engine,  a  supply  of 
gas  is  always  on  hand  for  quick  starting,  and  the  engines 
can  be  run  independently  of  the  rate  at  which  the  producer 
is  generating  gas. 

The  general  way  in  which  the  suction  gas-producer  operates 
is  shown  in  Fig.  19.  A,  the  producer  proper,  consists  of  a 
steel  shell  lined  with  firebrick,  and  is  provided  with  a  fire- 
grate, ash-pit,  etc.  The  producer  is  charged  with  coal  from 


FIG.  19. 

above  and  a  fire  started  at  the  bottom.  As  the  mass  of  coal 
above  the  fire  becomes  heated  gases  are  driven  off,  and 
when  the  engine  is  running  it  sucks  these  gases  into  the 
cylinder  and  burns  them  in  the  usual  manner.  When  the 
gases  leave  A  they  are  at  a  high  temperature  and  contain 
many  impurities.  They  first  pass  through  the  boiler  B 
where  they  are  cooled  by  water  circulating  through  vertical 
pipes.  The  hot  gases  generate  steam  in  B  and  some  of 
this  steam  passes  with  the  air  through  the  fire  in  A,  where 
it  helps  to  control  the  combustion  by  lowering  the  tempera- 
ture and  where  it  also  enriches  the  gas.  From  B  the  gases 
pass  through  a  wet  scrubber  C.  This  scrubber  contains 
several  layers  of  coke  on  which  water  is  sprayed  contin- 
4 


50  GAS-ENGINE  THEORY   AND   DESIGN 

uously.  The  object  is  to  further  cool  the  gases  and  remove 
impurities  such  as  tar,  pitch,  etc.,  which  sink  to  the  bottom 
of  C.  It  is  important  that  the  gases  should  be  as  clean  as 
possible  before  entering  the  engine  cylinder  as  impurities 
will  cause  the  piston  and  valves  to  stick  and  wear  rapidly. 
From  C  the  gases  pass  through  the  dry  scrubber  D  which 
contains  excelsior,  removes  further  impurities,  and  also 
prevents  pulsations  of  the  engine  from  reaching  A.  From 
D  the  gases  pass  to  the  engine. 

The  chief  problem  in  connection  with  producer  gas  is 
the  proper  cleaning  of  the  gas.  As  mentioned  in  the  fore- 
going, when  the  gas  leaves  the  producer  proper  (A)  it 
carries  along  tar,  ammonia,  sulphur,  dust,  etc.  In  order  to 
avoid  stoppages  and  irregular  running  of  the  engine  due 
to  clogging  up  and  wear  of  parts,  the  gas  should  be  dry  and 
froe  from  dust  and  other  impurities,  and,  of  course,  cool  in 
order  that  the  volume  may  be  reduced  to  a  minimum.  The 
design  of  producers  and  cleaning  apparatus  is  rapidly  chang- 
ing, and  it  will  doubtless  be  some  time  before  anything 
like  a  standard  of  construction  is  reached. 

The  use  of  hard  coal  in  the  producer  does  not  now 
present  any  difficulties  which  cannot  be,  or  rather  which 
have  not  been,  successfully  mastered.  The  use  of  soft 
coal,  on  account  of  tar  in  the  gas,  the  tendency  to  cake 
and  form  slag  and  adhere  to  the  sides  of  the  producer,  is 
more  difficult.  Doubtless  distinct  types  of  producers  for 
the  various  kinds  of  fuels  will  eventually  be  evolved.  The 
double- zone  producer,  a  description  of  which  can  be  found 
in  books  on  gas-producers,  furnishes  a  gas  free  from  tar. 

One  advantage  of  the  producer  which  cannot  be  over- 
estimated is  that  the  poorest  kinds  of  fuels  can  be  used. 
Among  the  fuels  successfully  used  are  lignite,  peat,  wood, 
straw,  mine  culm,  garbage,  and  many  waste  products. 

71.  CHEMICAL    REACTIONS     IN    THE    PRODUCER. — The 


FUELS  51 

chemical  reactions,  both  in  the  producer  and  in  the  engine 
cylinder,  are  really  very  complex,  depending  upon  a  number 
of  ever- varying  conditions  such  as  composition  of  the  fuel, 
amount  of  air  and  steam  supplied,  temperature  of  combus- 
tion, etc. 

Ihe  air  first  burns  to  C02,  then,  when  this  gas  strikes 
the  carbon  above  where  no  O  reaches  it,  it  is  decomposed 
into  CO.  When  steam  is  passed  through  the  fire  the  O 
separates  from  the  H  and  combines  with  carbon,  forming 
CO.  The  CO  and  H  then  pass  along  with  the  other  gases 
to  the  engine. 

When  carbon  burns  to  carbon  monoxide  the  following 
reactions  take  place: 
C  +  0  =  CO 
12  +  16  =  28 

16/ 12  X 100/23  =  weight  of  air  required. 
The  products  of  combustion  are: 

N 4.43  Ibs. 

CO..  .  2.34    " 


Total 6.78  Ibs. 

Now,  2.34  Ibs.  of  CO  burning  to  C02  furnish  4,320x2.34 
=  10,080  B.T.U. 

Carbon  burning  to  C02  furnishes . . .   14,500  B.T.U. 
CO  burning  to  C02  furnishes 10,080    " 

The  loss  equals 4,420  B.T.U. 

or  about  30  per  cent. 
The  heat  absorbed  in  separating  1  Ib.  of  H  from  the  0  in 

CO    OQA 

H20  is  53,300  B.T.U.,  then  -  -  =  5.33  Ibs.  carbon 

43.20  X  2.33 

required,  or  12.4  Ibs.  of  CO.  One  pound  of  CO  burning  to 
CO2  requires  about  2 J  Ibs.  of  air,  and  12.4  Ibs.  require 
31  Ibs.  of  air.  In  place  of  this  we  now  have  1  Ib.  of  H  of 


62 


GAS-ENGINE  THEORY  AND   DESIGN 


high  calorific  power,  with  a  decrease  in  the  heat  loss  of 
about  15  per  cent.  The  amount  of  steam  that  can  be  used, 
however,  is  limited,  since  if  the  temperature  of  the  fire  is 
lowered  too  much  the  H20  will  not  be  broken  up  and  a  loss 
instead  of  a  gain  results.  The  limit  of  the  ratio  of  steam 
to  coal  by  weight  is  about  1  to  40. 

The  producer  gas  carries  about  85  per  cent  of  the  calo- 
rific power  of  the  coal.    An  average  analysis  is  as  follows: 


By  Volume 
Hard  Coal        Soft  Coal 


CO 

H... 

CH< 

CH2 

N... 


27% 

12 
1.2 
2.5 

57.3 


27% 

12 
2.5 
2.0 

56.5 


80 


Producer  gas  contains  110-150  B.T.U.  per  cu.  ft 
cu.  ft.  of  gas  should  be  furnished  by  1  Ib.  of  coal. 

The  amount  of  coal  burned  to  C02,  and  which  furnishes 
the  heat  required  to  operate  the  producer,  is  usually  5 
per  cent  by  weight  of  the  fuel  consumed.  The  loss  in  radia- 
tion, ashes,  tarry  products,  etc.,  may  run  up  to  10  per  cent  in 
some  cases. 

The  properties  of  various  fuel  gases  are  given  in  Table  III. 

71a.— TABLE  III 

PROPERTIES    OF    FUEL   GASES 


B.T.U. 

WeiKht 

Cu.  Ft.  Air 

B.T.U. 

per  Cu. 
Ft. 

per  Cu. 
Ft. 

per  Cu.  Ft. 
Fuel 

per  Cu. 
Ft.  Mix- 
ture 

Symbol 

Natural  Gas  .... 

1,000 

0.045 

11 

85 

Coal  Gas  

650 

0.041 

8 

81 

Water  Gas  

630 

0.052 

6              90 

Oil  Water  Gas  

1,000 

0.062 

9-10 

100 

Blast-Furnace  Gas  

100 

0.085 

.80-1 

55 

Producer  Gas  

110-150 

0.072 

.90-1.25 

60-70 

Ethvlene  or  Olefiant  Gas. 

1,500 

0.076 

14.3 

100 

rViii 

Methane  or  Marsh  Gas.  .  . 

930 

0.043 

9.5 

90 

CH4 

Acetylene  

1,550 

0.081 

14 

110 

CaH2 

FUELS  53 

716. — ENRICHMENT  OF  FUELS. — Numerous  attempts 
have  been  made  to  enrich  the  liquid  fuels.,  i.  e.,  to  increase 
their  heating  value  per  pound,  by  adding  various  sub- 
stances. Thus  far  but  little  progress  has  been  made  in  this 
direction.  In  Germany  fair  results  have  been  obtained  with 
benzol,  C6H6  (coal-tar  benzine),  which  is  generally  mixed 
with  gasolene  in  varying  quantities.  The  object  is  to  in- 
crease the  heating  value  of  the  fuel  with  but  little  or  no 
increase  in  cost. 

72.  CALORIMETRY. — A  calorimeter  is  an  apparatus  for 
determining  the  calorific,  or  heating,  power  of  a  fuel. 

In  the  Mahler  calorimeter  the  liquid,  or  solid,  fuel  is  put 
into  a  vessel  filled  with  oxygen;  this  vessel  is  placed  in 
another  containing  water  so  that  it  is  entirely  surrounded 
by  water,  the  fuel  is  then  electrically  ignited  and  the  heat 
resulting  from  combustion  (which  is  complete,  as  an  excess 
of  oxygen  is  provided)  is  absorbed  by  the  water.  The  rise 
"in  the  temperature  of  the  water,  the  weight  of  which  is 
known,  plus  the  heat  absorbed  by  the  closed  vessel  (which 
amount  is  determined  beforehand)  gives  the  calorific 
power  of  the  fuel. 

The  Junker  gas  calorimeter  consists  of  a  cylinder  con- 
taining a  burner,  similar  to  the  Bunsen  burner,  through 
which  air  and  gas  flows.  The  hot  gases  pass  out  through 
tubes  surrounded  by  water — the  tubes  and  water  jacket  are 
outside  the  first  cylinder — and  the  cubic  feet  of  gas  consumed 
in  a  given  time,  and  the  amount  of  water  heated  to  a  cer- 
tain temperature,  will  give  the  calorific  power  of  the  fuel. 

There  are  also  a  number  of  calorimeters  in  which  chem- 
icals are  used  in  the  determination  of  the  calorific  power  of 
the  fuel.  These  possess  a  number  of  advantages  over  the 
older  forms  described  above. 

It  is  sometimes  desirable  to  analyze  exhaust  gases,  and 
chemical  calorimeters  for  this  purpose  are  on  the  market. 


CHAPTER  VII 

LAWS   OF   GASES 

73.  THE  INDICATOR  DIAGRAM.— In  Fig.  20  the  vertical 
lines,  called  ordinates,  represent  pressures,  and  the  hori- 
zontal lines,  called  abscissas,  represent  volumes.  0  is 
the  point  of  no  pressure  and  no  volume. 

Let  us  assume  that: 

(a)  The  piston  of  a  gas  engine  compresses  its  charge  of 
air  and  gas  from  a  to  b  without  an  increase  in  pressure ; 

(b)  The  mixture  is  then 
ignited  while  the  piston  is 
on  its  inner  dead  centre  and 
the    pressure    rises   during 
combustion  from  b  to  c; 

(c)  The  piston  now  moves 
out  and  enough  heat  is  sup- 
plied  to  keep  the   pressure 
constant,  giving  the  line  cd; 

(d)  At    the    end    of    the 

stroke  the  exhaust  valve  opens  and  the  pressure  in  the 
cylinder  falls  from  d  to  a,  then 

The  total  pressure  on  the  piston  in  pounds,  multiplied 
by  the  distance  ab  in  feet,  equals  the  work  done  in  foot- 
pounds. 

Therefore  the  area  abed  represents  the  work  done  by  the 
piston  on  its  out  stroke.  Such  a  diagram  is  called  an  in- 
dicator diagram. 

The  actual  indicator  diagram  is  very  different  from  the 
54 


LAWS  OF  GASES 


FIG.  21. 


above  in  form,  although  the  principles  are  the  same,  since 
it  is  impossible  to  compress  at  constant  pressure — and  con- 
sequently along  a  straight  line — and  to  expand  a  gas  in 
the  same  way.  Fig.  21  shows  the  form  of  an  actual  indi- 
cator diagram.  The  line  0V 
represents  zero  pressure,  i.e., 
14.7  Ibs.  below  atmospheric 
pressure.  The  total  pressure 
from  zero  is  called  absolute 
pressure,  and  is  measured 
in  either  pounds  per  square 
inch,  or  in  atmospheres — 1 
atmosphere  being  equivalent 
to  14.7  Ibs.  The  line  oaf  re- 
presents the  atmospheric  pressure.  This  indicator  diagram 
is  read  as  follows: 

(a)  The  piston  starts  to  compress  the  charge  at  a  and  the 
pressure  rises  during  compression  to  the  point  6; 

(6)  The  charge  is  now  ignited,  and  as  the  piston  starts 
to  move  out  the  pressure  rises,  due  to  the  combustion  going 
on,  to  the  point  c,  and  from  there  on  it  falls  as  the  com- 
bustion stops  and  the  volume  increases; 

(c)  At  the  point  d  the  exhaust  valve  opens  and  in  rush- 
ing out  the  gases  expand  to  atmospheric  pressure. 

The  area  enclosed  by  the  irregular  outline  abed  repre- 
sents the  work  done  by  the  burning  charge  in  expanding, 
just  as  the  area  in  Fig.  20  represents  the  work  done.  This 
area  in  square  inches,  divided  by  the  distance  ab,  gives  the 
mean  height  of  a  rectangle  having  the  same  area  as  the 
diagram,  as  indicated  by  the  dotted  lines.  If,  for  example, 
the  scale  of  pressure  is  200  Ibs.  per  inch,  and  this  mean 
height  is  \  inch,  then  the  mean  pressure  throughout  the 
stroke  is  100  Ibs.  This  mean  effective  pressure  is  abbre- 
viated M.E.P. 


5(5  GAS-ENGINE  THEORY  AND   DESIGN 

The  work  done  by  the  expanding  charge  is  now  figured 
as  follows: 

PLAN 

=  "33^000" 
where  P=M.E.P. 

A  =  area  of  piston  in  sq.  in. 
L  =  stroke  of  piston  in  feet. 
N  =  R.P.M.  (power  strokes). 

In  a  single-cylinder  four-cycle  engine  the  power  strokes 
are  only  one-half  of  the  total  R.P.M. 

The  planimcter  is  an  instrument  for  measuring  the  area 
of  a  surface  having  an  irregular  outline  like  the  indicator 
diagram. 

The  shaded  portion  in  Fig.  21  represents  the  work  done 
by  the  fly-wheel  during  compression.  This  is  again  restored 
to  the  fly-wheel  by  the  gas  during  expansion  and  does  not 

affect  the  indicator  diagram 
proper,  and  so  is  neglected 
in  our  present  calculations. 

Fig.  22  is  an  indicator 
diagram  from  a  four-cycle 
engine.  During  the  exhaust 
stroke  the  pressure  in  the 
cylinder  rises  a  little  above 

v  ^ 

1>9  atmospheric    due    to    back 

pressure  of  the  gases,  and 

during  the  suction    stroke   the   pressure  drops  somewhat 
below  atmospheric  due  to  wire-drawing  effect. 

74.  THE  INDICATOR. — Fig.  23  illustrates  the  principles 
on  which  the  indicator  works.  The  card  C,  on  which  the 
indicator  diagram  is  drawn  by  the  pencil-point  T,  moves 
back  and  forth  to  correspond  with  the  movement  of  the 
engine  piston  P.  The  cylinder  Df  communicates  with  the 
engine  cylinder  D.  In  D'  there  is  a  small  piston  P'  (area 


LAWS  OF  GASES 


1  sq.  in.)  which  works  against  a  stiff  spring  so  graduated 
that  it  registers  on  C  the  pressure  against  pistons  P  and  Pf 
to  some  definite  scale.  If  a  pressure  of  100  Ibs.  will  com- 
press the  spring  I",  then  a  point  on  the  indicator  curve  1" 
above  atmospheric  press- 
ure shows  that  the  press- 
ure in  the  piston  was 
100  Ibs.  at  that  instant, 

Now  it  can  easily  be 
seen  that  as  P  moves  back 
and  forth,  P'  moves  up 
and  down,  and  the 
changes  of  pressure  and 
volume  in  D  and  D'  are 
traced  on  C  by  the  point 
T.  The  indicator  card 
thus  furnishes  a  record 
of  the  change  of  pressure 
and  volume  in  the  engine 
cylinder,  and  consequently  of  the  work  done.  The  horse- 
power computed  from  the  indicator  diagram  is  called  the 
indicated  horse-power  and  is  abbreviated  I.H.-P. 

The  actual  indicator  carries  a  drum  on  which  the  card 
is  mounted,  and  instead  of  moving  back  and  forth  this 
drum  revolves  through  a  certain  angle.  A  reducing  mech- 
anism is  provided  between  the  drum  and  piston  rod,  since 
the  movement  of  the  drum  is  very  small  compared  with 
the  piston  travel. 

75.  CHANGES  IN  A  GAS. — The  state  of  a  gas  may  be 
changed  by:  adding  to  or  subtracting  heat  from  it;  doing 
external  work  upon  it.  This  will  bring  about  changes  in: 
volume,  temperature,  pressure,  specific  heat,  intrinsic 
energy,  entropy. 

In  the  following  pages  a  permanent  gas,  air,  will  be  con- 


FIG.  23. 


58  GAS-ENGINE  THEORY  AND   DESIGN 

sidered,  but  the  laws,  with  the  substitution  of  the  proper 
constants,  apply  to  all  gases. 

Certain  laws  regarding  changes  in  pressure,  volume,  and 
temperature  of  gases  have  been  determined  by  experiment, 
and  these  will  be  briefly  mentioned. 

76.  LAW  OF  GAY-LUSSAC  OR  CHARLES:    VOLUME  AND 
TEMPERATURE.  —  The  pressure  remaining  constant,  the  vol- 
ume of  a  perfect  gas  is  proportional  to  its  absolute  tem- 
perature. 

V,  =  T, 
Vt      T, 

or,  the  volume  remaining  constant,  the  pressure  must  vary 
directly  with  the  absolute  temperature1 
P,=  T, 
PI      T, 

also,  V,  =  V0(H-at) 

where  T  =  abs.  temp. 

V  =  volume  in  cu.  ft. 
P  =  pressure  in  pounds  per  sq.  ft. 
a  =  1/493  =  0.002035. 
t  =  temp,  above  32°. 

The  volume  of  a  perfect  gas  increases  1/493  for  each 
degree  increase  in  temperature. 

77.  LAW  OF  MARIOTTE  OR  BOYLE:   VOLUME  AND  PRESS- 
URE. —  The  temperature  remaining  constant,  the  volume 
must  vary  inversely  as  the  pressure,  and  directly  as  the 
density  (since  density  varies  inversely  with  the  volume). 


78.  COMBINING  OF  LAWS.—  A  combination  of  the  fore- 
going laws  gives: 


PV     P.V,     P2V2 


LAWS  OF  GASES 

=  K,  a  constant. 


T  Tt  T2 

PV  =  TK 

Note. — The  mathematical  proof  for  the  above,  and  a 
number  of  other  formulas,  will  not  be  given,  but  can  be 
found  in  books  on  thermodynamics. 

For  air  K,  12.39  X  (144X14.7)^ 

493 
ternal  work  done  for  each  degree  increase  in  temperature. 

If  we  had  a  closed  cylinder  containing  12.39  cu.  ft  of  air 
(1  Ib.)  at  atmospheric  pressure,  behind  a  piston  having  an 
area  of  1  sq.  ft.  (144  sq.  in.),  then,  if  this  body  of  air  is  heated 
so  that  the  temperature  rises  1°,  its  volume  will  increase 
1/493  of  12.39  cu.  ft,  and  the  piston  will  be  pushed  forward 
1/493  of  12.39  linear  feet  against  a  pressure  of  144  X  14.7  Ibs. 

The  value  of  K  changes  for  different  substances,  for 
example: 

For  superheated  steam  K  =  104.64. 
"    ammonia  K  =  162.60. 

79.  INTRINSIC  ENERGY. — In  order  to  have  a  gas  do  work 
it  must  be  heated,  and  it  can  be  seen  from  the  above  that 
the  capacity  of  a  gas  for  doing  work  depends  upon  its 
specific  heat,  its  weight,  and  its  absolute  temperature; 
therefore  intrinsic  energy  =  G  C  T 

where  G  =  weight  in  Ibs. 
C  =  specific  heat 
T  =  abs.  temperature. 

80.  AVAILABLE  ENERGY. — Since,  in  order  to  expand,  the 
gas  must  do  work  against  atmospheric  pressure,  the  energy 
expended  in  doing  this  work  is  measured  by  the  product 
of  the  weight  of  the  gas  X  its  specific  heat  X  the  difference 
between  the  initial  and  final  absolute  temperatures,  or 

Available  energy  =  G  X  C  X  (Tt  -T2). 


60 


GAS-ENGINE   THEORY   AND    DESIGN 


81.  EXPANSION. — The  expansion  may  be  isopiestic,  iso- 
thermal, adiabatic,  according  to  the  law  PVn=a  constant. 

82.  ISOTHERMAL  EXPANSION  takes  place  at  constant  tem- 
perature.   When  a  gas  expands  under  ordinary  conditions 
its  temperature  falls.     In  order  to  expand  isothermally 
heat  would  have  to  be  supplied.    In  Fig.  24  let  the  curve 


represent  the  isothermal  expansion  of  a  gas  from  a  to  b, 
then  V  =the  initial  volume, 

V1=the  final  volume, 

P  =the  initial  pressure, 

P^the  final  pressure. 

Since  the  temperature  does  not  change  the  expansion 
follows  the  law 


The  isothermal  curve  is  an  equilateral  hyperbola  and  is 
expressed  as  follows: 


where  AY  =  the  work  done,  log  e  is  the  hyperbolic  log. 

83.  ADIABATIC  EXPANSION.—  In  expanding  adiabatically 
a  gas  does  not  receive  heat  from,  or  give  out  heat  to,  any 
external  body.  The  external  work  done  during  expansion 


LAWS   OF   GASES 


61 


is  done  at  the  expense  of  the  intrinsic  energy  of  the  gas. 
There  is  a  fall  in  temperature  and  pressure. 

In  Fig.  25  let  the  curve  ab  represent  isothermal  expan- 
sion, and  the  curve  ac  adiabatic  expansion.  Since  no  heat 
is  added  during  adiabatic  expansion  the  final  pressure  will 
be  lower  than  for  isothermal  expansion.  The  PV  of  the 


FIG.  26. 


FIG.  27. 


isothermal  expansion  becomes  PVn  in  adiabatic  expansion. 
The  n  is  the  ratio  of  the  two  specific  heats  of  the  gas  and 
expresses  the  ratio  of  the  change  in  pressure  and  volume 
in  adiabatic  expansion. 

Isothermal  expansion  would  be  more  wasteful  than  adia- 
batic since  the  gases  would  be  exhausted  at  a  much  higher 
temperature. 

8i.  ISOMETRIC  LINES. — In  Fig.  26  the  line  be  represents 
an  increase  in  pressure  without  an  increase  in  volume. 
Such  a  rise  in  pressure  would  take  place  when  a  piston  is 
held  stationary  while  the  charge  explodes. 

85.  ISOPIESTIC  LINES  OR  ISOBARS. — In  Fig.  27  the  line 
be  represents  an  increase  in  volume  without  a  change  in 
pressure.    Such  lines  are  called  isobars. 

86.  SPECIFIC  HEAT  AT  CONSTANT  VOLUME  AND  AT  CON- 
STANT PRESSURE. — Specific  heat  has  already  been  defined 
as  the  amount  of  heat  necessary  to  raise  the  temperature  of 


02  GAS-ENGINE  THEORY   AND   DESIGN 

a  unit  weight  of  a  substance  1°.  When  air  is  heated  and 
allowed  to  expand  a  certain  amount  of  heat  is  necessary 
to  raise  the  temperature  1°  under  these  conditions.  If  the 
same  amount  of  air  is  confined  in  a  closed  vessel  so  that  it 
cannot  expand,  and  the  same  amount  of  heat  is  applied, 
the  temperature  will  obviously  be  higher  than  in  the  first 
case.  A  gas  therefore  has  two  specific  heats.  Experiment 
has  shown  that  the 

Specific  heat  of  air  at  constant  pressure  (Cp)  is  0.2375. 

Specific  heat  of  air  at  constant  volume   (Cv)  is  0. 1691. 

Cp     .2375 

—  = =  1.405 =n,  a  constant. 

For  isothermal  expansion  n  =  1. 

For  adiabatic  n  =  1.405. 

One  Ib.  of  air  raised  1°  at  constant  pressure  requires 
778  X  .2375  - 184.77  ft.-lbs. 

One  Ib.  of  air  raised  1°  at  constant  volume  requires 
778 X.  1695  =  131.56  ft.-lbs. 

The  real  specific  heat  of  the  products  of  combustion  is  a 
very  uncertain  quantity  and  differs  more  or  less  from  the 
theoretical  specific  heats.  It  can  be  approximated  by  an 
analysis  of  the  products  of  combustion — by  taking  the 
mean  of  the  various  specific  heats.  In  Table  IV.  the  specific 
heats  and  volumes  of  some  gases  are  given. 

87.  EXPANSION  ACCORDING  TO  THE  LAW  PVn  =  A  CON- 
STANT.— Isothermal  and  adiabatic  expansions  are  possible 
only  theoretically  and  can  never  be  realized  in  practice. 
They  are  very  useful  in  the  development  of  the  theory  of 
thermodynamics.  The  actual  expansion  of  a  gas  takes 
place  according  to  the  formula  PVn  =  a  constant,  in  which 
n  is  the  ratio  of  the  specific  heats  of  the  gas.  This  formula 
may  be  written  as  follows: 

PVn  =  P1V1n  and     -= 


LAWS  OF  GASES  63 

or,  the  pressure  varies  inversely  with  the  nth  power  of  the 
volume. 

In  practice,  for  obvious  reasons,  the  value  of  n  will  vary 
more  or  less  from  the  theoretical  value  1.405  (for  air),  but 
the  actual  value  of  n  for  an  engine  under  given  conditions 
can  be  computed  from  its  indicator  card. 

The  actual  compression  curve  on  the  indicator  card  will 
usually  lie  between  the  isothermal  and  adiabatic  curves 
(see  Fig.  25),  and  follows  the  law  PVn  =  K,  the  value  of  n 
being  1.35  or  1.33. 

88.  COMPRESSION. — The  laws  and  formulas  for  expansion 
apply  equally  well  to  compression  since  compression  is 
simply  the  reverse  of  expansion. 

89.  COMPRESSION  IN  Two. STAGES. — The  work  of  com- 
pression is  lessened  if  the  work  is  carried  on  in  two,  or  more, 
stages  so  that  the  air  can  be  inter-cooled;    for  example, 
a  body  of  air  is  to  be  compressed  to  10  per  cent  of  its  orig- 
inal volume.    If  it  is  compressed  in  the  first  stage  (first  cyl- 
inder) to  55  per  cent,  the  temperature  will  have  increased 
to  something  like  400°.    On  its  way  to  the  second  cylinder 
the  air  is  cooled  back  to  its  initial  temperature,  say  70°,  and 
now  much  less  energy  is  required  to  compress  it  from  55 
per  cent  to  10  per  cent  of  the  original  volume  than  would 
have  been  the  case  if  the  heat  had  not  been  withdrawn. 

90.  THE  CARNOT  CYCLE. — The  term  " cycle"  here  refers 
to  a  succession  of  heat  changes  in  the  gas. 

In  the  theoretical  cycle  of  maximum  efficiency  proposed 
by  Carnot  it  is  assumed  that: 

(a)  There  is  a  heat  reservoir  of  unlimited  capacity  so 
that  heat  can  be  supplied  without  a  change  in  temperature ; 

(6)  There  is  a  refrigerator  of  unlimited  capacity  so  that 
heat  can  be  withdrawn  without  a  change  in  temperature; 

(c)  The  engine  cylinder  and  piston  are  non-conducting 
so  that  heat  cannot  escape  that  way; 


r,4 


GAS-ENGINE  THEORY   AND   DESIGN 


(d)  The  engine  is  connected  to  both  heat  reservoir  and 
refrigerator  so  that  heat  can  be  received  and  discharged. 
The  cycle  then  operates  (Fig.  28)  as  follows: 

(a)  The  engine  is  connected  to  the  heat  reservoir  and  heat 
flows  into  the  cylinder  so  that  the  gas  expands  isothermally 
from  c  to  d; 

(b)  The  connection  is  closed  and  the  expansion  continues 
adiabatically  from  d  to  a; 


FIG.  28. 

(c)  The  engine  is  now  connected  with  the  condenser  which 
withdraws  heat  isothermally  while  the  piston  moves  from 
a  to  b; 

(d}  The  connection  to  the  condenser  is  closed  and  the  gas 
is  compressed  adiabatically  from  b  to  c  so  that  when  the 
point  c  is  reached  the  gas  is  in  exactly  the  same  condition 
as  at  the  beginning  of  the  stroke. 

All  heat  transfers  have  been  made  at  maximum  efficiency 
so  that  the  efficiency  of  the  cycle  is  expressed  as  follows: 

T  —  T, 

—  =  Efficiency. 


This  cycle  is  reversible.  It  is  assumed,  of  course,  that 
cylinder  and  piston  are  insulated  so  that  they  will  not  al> 
sorb  any  heat. 


LAWS  OF   GASES 


or, 


The  actual  gas-engine  cycles  necessarily  differ  greatly 
from  the  Carnot  since  the  latter  imposes  conditions  which 
can  never  be  realized  in  practice,  but  it  points  out  the  lines 
along  which  the  greatest  thermal  efficiency  could  be  secured. 

Other  cycles,  such  as  the  Beau  de  Rochas,  Lenoir,  Bray- 
ton,  Diesel,  etc.,  have  already  been  described. 


91. -TABLE  IV 

VOLUMES   AND   SPECIFIC   HEATS   OF  GASES 


Specif] 

cHeat 

Gas 

Vol.  at  32° 

Constant 
Pressure 

Constant 
Volume 

Air  

12.39 

0.2375 

0   1690 

Carbon  monoxide  

12.77 

0  2479 

0   1758 

Carbon  dioxide  

8  12 

0  2170 

0  1535 

Hydrogen.  .  .      .          

178  80 

3  4090 

2  4122 

Nitrogen.  .              .                  .... 

12  77 

0  2438 

0  1727 

Oxygen.  .  . 

11  20 

0  2175 

0  1550 

Superheated  steam. 

0  4805 

0  3460 

Alcohol. 

0  4534 

0  3200 

Ammonia  . 

0  508 

0  299 

CHAPTER  VIII 

GAS-ENGINE   EFFICIENCY 

92.  In  a  discussion  of  the  efficiency  of  the  gas  engine 
certain  factors  must  be  considered.     Among  the  factors 
to  be  discussed  are:  reliability,  economy,  advantages,  dis- 
advantages. 

93.  RELIABILITY. — The    desirability    of    installing    gas 
power  depends  much  upon  the  reliability  of  the  engine, 
and  into  the  question  of  reliability  enter  a  number  of  con- 
siderations, among  which  are  cost,  overload  and  underload 
capacity,  proper  handling,  etc. 

The  only  way  to  improve  the  design  of  any  machine  is 
to  acknowledge  its  faults  and  then  work  to  overcome  them. 
The  advantages  of  the  gas  engine  are  many;  its  disad- 
vantages should  be  carefully  studied  with  a  view  to  min- 
imizing or  overcoming  them  altogether.  In  the  matter  of 
reliability,  there  is  still  much  room  for  improvement,  espe- 
cially in  connection  with  some  of  the  apparatus  connected 
with  the  operation  of  the  engine.  Without  reliability  any 
heat  engine  is  of  little  use.  Gas-engine  design  presents  a 
much  more  difficult  problem  than  the  steam  engine,  for  a 
number  of  reasons.  The  gas  engine  has  no  reservoir  in 
which  energy  can  be  stored;  the  power  is  delivered  inter- 
mittently to  the  crank-pin;  high  temperatures,  pressures, 
and  fuel  conditions  make  a  reliable  performance  under 
widely  varying  conditions  something  that  will  result  only 
from  careful  designing,  good  workmanship,  and  proper  hand- 
ling. It  has  been  customary  on  the  part  of  some  manufac- 
66 


GAS-ENGINE   EFFICIENCY  67 

turers,  especially  of  the  smaller  engines,  to  lead  the  cus- 
tomer to  believe  that  a  gas  engine  will  run  itself  without 
care  and  attention.  This  is  far  from  being  the  case.  Gas 
engines,  both  large  and  small,  must  be  handled  intelligently 
in  order  to  give  satisfactory  results.  There  is  no  reason, 
however,  why  a  fairly  intelligent  man  possessing  some 
mechanical  ability  should  not  become  competent  to  handle 
a  producer-gas  plant,  for  example,  with  a  reasonable  amount 
of  training.  When  a  power-user  installs  a  steam  plant  he 
knows  perfectly  well  that  he  must  have  competent  men  to 
take  care  of  it,  and  it  is  just  as  important  that  a  gas  engine, 
no  matter  whether  large  or  small,  should  receive  proper 
attention,  although  the  help  required  is  less  for  a  gas- 
power  plant  than  for  a  steam-power  plant. 

The  user  of  steam  power  knows  that  he  can  buy  a  cheap 
engine  and  boiler  outfit  and  will  get  exactly  what  he  pays 
for.  He  also  knows  that  if  he  wants  a  thoroughly  reliable 
steam  plant,  especially  where  the  conditions  are  very 
exacting,  he  must  pay  accordingly.  He  knows  that  he 
cannot  get  a  high-class  steam-power  outfit  at  bargain  prices. 
Precisely  the  same  conditions  hold  good  for  the  gas  engine. 
If  a  power-user  wants  a  good  reliable  gas-power  outfit,  no 
matter  whether  large  or  small,  he  must  pay  accordingly 
and  see  that  proper  care  is  taken  of  the  plant  after  installa- 
tion. A  cheap  engine  cannot  be  expected  to  be  reliable, 
or  to  develop  the  full  power  at  which  it  is  rated.  Nor  will 
it  run  without  attention,  no  matter  what  an  over-anxious 
salesman  may  say.  Much  trouble  has  been  caused  in  the 
past  by  poorly  designed  and  cheaply  constructed  engines 
which  would  look  better  in  a  museum  than  in  a  shop.  In 
connection  with  producer  and  blast-furnace  gas  engines 
much  trouble  has  been  caused  by  improper  cleaning-ap- 
paratus for  the  gas.  In  order  that  the  engine  may  work 
successfully  the  entire  plant  must  work  successfully.  An 


68  GAS-ENGINE  THEORY   AND   DESIGN 

engine  cannot  be  expected  to  properly  perform  its  work 
when  improper  fuel  is  delivered  to  it.  Too  many  engines 
and  producers  have  been  installed  without  having  been 
properly  tested. 

Where  power  only  is  wanted  the  gas  engine  has  much  in 
its  favor,  but  where  steam  is  needed  for  heating  during  the 
winter,  or  for  manufacturing  purposes — as  in  paper  mills, 
textile  mills,  etc.,  a  special  heating  system  would  have  to 
be  installed  in  connection  with  the  gas  engine,  and  under 
these  conditions  the  steam  engine  possesses  some  advan- 
tages. In  the  large  electric  light  and  power  plants,  on  the 
other  hand,  a  vast  amount  of  heat  is  wasted  in  exhaust 
steam,  and  this  waste  could  be  largely  avoided  by  the  use 
of  a  high-grade  gas  engine.  Attempts  have  been  made  to 
use  the  exhaust  gases  of  a  gas  engine  for  heating  purposes, 
or  generating  steam,  and  it  is  claimed  that  10  per  cent  of 
the  heat  has  been  saved  in  this  way. 

The  steam  engine  will  carry  a  large  overload  and  will  pull 
hard  under  varying  loads,  since  it  has  a  large  reservoir  of 
energy  to  draw  from.  In  the  case  of  the  gas  engine  the 
obstacles  to  be  overcome  in  order  to  achieve  the  same 
results  are  great.  Unless  a  gas  engine  is  designed  for  an 
overload  capacity  it  will  simply  slow  down  and  stop  when 
overloaded  to  any  extent,  and  while  it  may  be  very  efficient 
under  full  load  it  may  drop  greatly  in  efficiency  when  run- 
ning under  three-quarter  or  half  load,  and  when  the  load  is 
much  less  than  one-half  it  may  again  stop.  A  gas  engine 
should  be  designed  first  for  reliability  under  the  conditions 
under  which  it  is  to  work. 

A  good  design  alone  will  not  produce  a  good  engine. 
Unless  the  work  in  the  shop  is  right  a  poor  engine  will 
result.  A  well-built  engine  of  poor  design  will  give  some 
results,  while  a  well  designed  but  poorly  built  engine  will 
never  give  results. 


GAS-ENGINE  EFFICIENCY  69 

94.  ECONOMY. — The  thermal  efficiency  of  different  gas 
engines  varies,  of  course,  with  the  type  of  engine  and  the 
goodness  of  the  design.  Automobile,  marine,  and  the 
smaller  engines  have  efficiencies  of  15  to  20  per  cent.  The 
large  gas  engines  have  efficiencies  ranging  from  15  to  30, 
and  even  35  per  cent  (brake  efficiency).  The  theoretical 
heating  value  of  a  fuel  can  never  be  realized  in  a  gas  engine, 
but  it  has  been  customary  to  figure  the  thermal  efficiency 
by  taking  the  theoretical  values  as  100  per  cent. 

The  heat  losses  are  about  as  follows:  Heat  absorbed  by 
the  water  jacket,  30-50  per  cent;  heat  converted  into 
work  (indicated),  15-40  per  cent;  heat  lost  in  the  exhaust, 
30-40  per  cent. 

From  the  heat  converted  into  work  must  be  subtracted 
the  work  done  in  overcoming  engine  friction,  which  may 
run  from  1|  to  10  per  cent,  and  the  remainder  will  be  the 
output  of  the  engine. 

The  water-jacket  loss  can  be  reduced  by  circulating  the 
water  slowly  so  that  the  temperature  of  the  cylinder  walls 
and  other  parts  is  kept  just  below  the  danger-point.  This 
is  a  difficult  thing  to  do  in  practice. 

The  heat  loss  in  the  exhaust  can  be  reduced  by  providing 
for  complete  combustion  and  early  ignition  so  that  the 
pressure  and  temperature  in  the  cylinder  will  be  as  low  as 
possible  when  the  exhaust  valve  opens.  The  efficiency  of 
the  engine,  in  general,  can  be  increased  by:  having  the 
greatest  piston  speed  practicable;  the  greatest  possible 
expansion;  increasing  the  compression  as  much  as  fuel 
conditions  will  allow;  rapid  and  complete  combustion; 
keeping  the  excess  of  air  over  that  required  for  combustion 
as  low  as  possible. 

It  may  be  stated  as  a  general  rule  that  economy  will 
increase  directly  with  the  compression.  A  striking  example 
of  this  is  found  in  the  Diesel  engine,  where  a  thermal 


70  GAS-ENGINE   THEORY   AND   DESIGN 

efficiency  of  38  per  cent,  or  more,  is  obtained  chiefly  by 
high  compression.  Another  example  is  found  in  the  Banki 
engine.  The  manner  in  which  the  fuel  is  handled  puts  a 
practical  limit  on  the  compression,  for  if  a  mixture  of  air 
and  gas  (or  vapor)  is  heated  by  compression  beyond  the 
ignition  temperature  it  will  ignite  too  early  and  a  back 
explosion  results.  If  the  fuel  is  forced  into  the  cylinder 
after  the  piston  has  completed  its  compression  stroke,  then 
the  degree  of  compression  is  limited  only  by  the  mechanical 
construction.  The  compression  limit  is  raised  in  the  larger 
engines  by  the  use  of  water-cooled  pistons,  valves,  etc. 

A  large  engine  consumes  less  fuel  per  H.-P.  hour  than  a 
small  engine.  Many  of  the  large  gas  engines  are  sold  under 
a  guarantee  to  develop  their  full  rated  B.H.P.  on  a  con- 
sumption of  10,000  effective  B.T.U.  per  H.-P.  hour,  and 
at  50  per  cent  loads  with  a  consumption  of  13,000  B.T.U. 
The  actual  consumption  of  some  engines  under  full  load  is 
8,500  B.T.U.  per  H.-P.  hour. 

Oil  engines  are  usually  guaranteed  to  develop  their 
rated  B.H.P.  on  a  consumption  of  1  Ib.  of  oil  per  H.-P. 
hour  under  full  load.  In  a  number  of  engines  of  more  than 
20  H.-P.  this  falls  to  j  Ib. 

The  performance  of  an  average  gas  engine  under  average 
conditions  is  very  different  from  that  of  an  engine  carefully 
adjusted  and  tested  by  an  expert  under  the  very  best 
possible  conditions. 

95.  ADVANTAGES  AND  DISADVANTAGES. — The  more  im- 
portant advantages  of  gas  engines  may  be  summed  up  as 
follows:  Small  space  occupied  as  compared  with  the  steam 
plant;  can  be  quickly  started  and  stopped  at  any  time; 
simplicity;  fuel  is  consumed  only  while  the  engine  is  run- 
ning; economy  of  fuel;  cheap  fuels  can  be  used;  com- 
paratively low  cost  of  upkeep  and  attendance. 

Among  the  disadvantages  are:    Inability  to  carry  over- 


GAS-ENGINE   EFFICIENCY  71 

loads  unless  specially  designed;  decreased  efficiency  when 
run  at  less  than  full  loads;  regulation  often  poor;  cannot 
be  started  under  load;  irreversibility. 

Most  of  the  disadvantages  can  be  overcome  to  a  great 
extent,  or  altogether,  by  good  design,  workmanship  and 
proper  handling. 

96.  MEDIA  USED  IN  HEAT  ENGINES. — Many  attempts 
have  been  made  to  use  media  other  than  air  and  steam  in 
heat  engines.    Experiments  too  numerous  to  mention  have 
been  made  with  alcohol,  chloroform,  ammonia,  naphtha, 
ether,  etc.,  but  the  results  have  been  unsatisfactory  for 
various  reasons.    The  advantage  of  a  low  specific  heat  is 
counterbalanced,  perhaps,  by  the  greater  weight  of  the 
medium  required  per  stroke.     Some  of  these  media  are 
expensive,  others  dangerous,  some  have  an  offensive  odor, 
others  are  explosive,  irrespirable,  etc.     Air  and  water  pos- 
sess two  immense  advantages:  they  are  abundant  and  safe. 

97.  OTHER  TYPES  AND  CYCLES  OF  HEAT  ENGINES. — The 
various  cycles,  such  as  the  Brayton,  Lenoir,  etc.,  have  al- 
ready been  mentioned.    The  limitations  of  the  compression 
cycle  have  been  pointed  out  in  the  preceding  paragraphs. 
The  other  types  include  hot-air  engines,  engines  using  media 
other  than  air  or  water,  etc.    Hot-air  engines,  in  which  a 
body  of  air  confined  in  a  closed  space  is  generally  heated 
from  the  outside,  have  been  used  to  a  limited  extent  for 
small  pumping  outfits,  but  in  larger  powers  are  too  bulky 
to  be  commercially  successful.    Naphtha  engines,  in  which 
naphtha  is  used  in  the  same  manner  as  steam  in  a  boiler, 
have  limited  application  for  small  marine  work.     Compound- 
ing, combinations  of  different  media  used  in  series,  etc.,  have 
been  tried,  but  found  to  be  impracticable.     In  order  to  be 
a  real  success  an  engine  must  be  a  commercial  success,  and 
if  it  does  not  answer  the  commercial  requirements  it  will 
be  a  failure,  no  matter  how  perfect  it  may  be  theoretically. 


CHAPTER  IX 

EXPLOSIVE   MIXTURES 

98.  P^xplosion  has  been  defined  as  extremely  rapid  com- 
bustion— as   practically   instantaneous   combustion.     The 
burning  of  the  charge  in  the  gas-engine  cylinder  is  usually 
so  rapid  that  the  term  "explosion"  is  commonly  applied 
to  this  combustion,  but  it  can  hardly  be  called  an  explosion 
in  the  sense  that  gunpowder  explodes,  since  the  combustion 
in  the  engine  cylinder  requires  an  appreciable  length  of  time. 

99.  COMPRESSION. — It  has  been  mentioned  in  the  pre- 
ceding chapter  that  the  economy  is  increased  by  increasing 
the  compression.     The  reasons  for  this  are  that  with  air 
compressed  into  a  small  space  the  combustion  is  more 
rapid  and  complete  than  with  low  or  no  compression,  the 
heat  has  a  better  opportunity  of  exerting  pressure  against 
the  piston  before  it  is  absorbed  by  the  cylinder  walls,  and 
increasing  the  compression  practically  means  lengthening 
the  stroke.    With  low  compression  only  a  part  of  the  fuel 
may  be  burned.    Some  gases  are  so  poor  that  they  will  not 
burn  at  all  with  low  compression,  and  for  this  reason  engines 
running  on  blast-furnace  gas,  for  example,  compress  to  !.">() 
or  even  200  Ibs.,  wrhile  engines  running  on  illuminating  gas 
may  compress  to  only  75  Ibs.    The  method  of  handling  the 
fuel,  however,  puts  a  practical  limit  to  the  allowable  com- 
pression. 

Note. — Sec  also  Cooling  by  Water  Injection. 

100.  METHODS  OF  HANDLING  FUEL. — There  are  prac- 
tically three  methods  of  handling  the  fuel,  viz. :  Compressing 
an  explosive  mixture  of  air  and  fuel  and  igniting  by  the 

72 


EXPLOSIVE  MIXTURES  73 

electric  spark;  compressing  air  only  and  injecting  the  fuel 
upon  completion  of  the  compression  stroke;  compressing 
the  air  and  injecting  the  fuel  into  a  vaporizing  chamber 
during  the  compression  stroke. 

The  simplest  method  of  handling  the  fuel  is  to  draw  a 
charge  of  air  and  gas  (or  vapor)  into  the  cylinder  during 
the  suction  stroke  and  then  compress  this  explosive  charge. 
This  mixture  must  be  kept  below  the  ignition  temperature 
during  compression  in  order  to  prevent  pre-ignition,  and 
this  is  the  one  great  disadvantage  of  the  method.  In  the 
larger  engines,  as  mentioned  before,  the  compression  limit 
is  raised  by  cooling  the  piston  and  valves  as  well  as  the 
cylinder  and  cylinder-head,  and  by  diluting  the  explosive 
mixture  with  an  excess  of  air. 

The  method  of  injecting  fuel  into  the  combustion  space 
after  the  compression  stroke  has  been  completed  permits 
the  compression  to  be  increased  as  much  as  the  mechanical 
construction  will  permit.  This  method  results  in  high  fuel 
economy  and  does  away  with  all  ignition  apparatus,  since 
the  temperature  of  the  compressed  air  is  high  enough  to 
ignite  the  fuel.  The  disadvantages  of  this  method  are 
greater  stresses  in  the  machine,  more  friction,  greater 
difficulty  in  starting,  and  the  extra  mechanism  necessary  for 
handling  the  fuel.  One  great  advantage  is  that  cheap  petro- 
leum oils  can  be  used,  and,  since  the  combustion  is  practi- 
cally complete,  no  carbon  deposits  in  the  cylinder  will  result. 

The  method  of  injecting  fuel  during  the  compression 
stroke,  vaporizing  and  igniting  this  fuel  by  means  of  a  hot 
chamber,  is  carried  out  in  the  Hornsby-Ackroyd  and  several 
of  the  two-cycle  oil  engines.  The  hot  chamber  furnishes 
the  necessary  heat  for  vaporizing  and  igniting  the  heavy 
oils.  The  advantages  of  this  method  are  ability  to  handle 
heavy  oils,  and  simplicity.  The  disadvantages  are  that 
carbon  deposits  cannot  be  prevented  (although  an  attempt 


74  GAS-ENGINE  THEORY  AND   DESIGN 

is  made  to  confine  them  to  the  combustion  chamber),  and 
the  time  required  for  starting. 

101.  SCAVENGING. — In   the   large   two-cycle   engines   a 
scavenging  charge  is  employed.     Separate   air  and  gas 
pumps  are  used.    As  the  burnt  gases  pass  out  through  the 
exhaust  port  a  charge  of  fresh  air  is  pumped  into  the  cyl- 
inder, and  following  this  comes  the  new  combustible  charge. 
The   scavenging  charge   clears  the  cylinder   of   all   burnt 
gases  and  also  prevents  prc-ignition,  since  the  hot  exhaust 
gases  cannot  come  in  contact  with  the  fresh  combustible 
charge.     In  the  four-cycle  engine  the  fresh  charge  cannot 
exceed  in  volume  the  piston  displacement,  and  a  body  of 
burnt  gases  equal  in  volume  to  the  compression  space  always 
remains  in  the  cylinder.    By  scavenging  and  pumping  in  the 
fresh  charge  the  entire  cylinder  is  rilled  with  an  explosive 
mixture  and  the  power  output  of  the  engine  is  increased. 

102.  DILUTION  OF  EXPLOSIVE  MIXTURES. — Experiments 
have  shown  that  combustion  is  more  rapid,  and  the  highest 
pressures  are  obtained,  when  the  volume  of  air  is  only 
slightly  in  excess  of  that  required  for  combustion.     An 
excess  of  either  air  or  gas  hinders  combustion  besides  mak- 
ing an  extra  amount  of  fluid  to  be  heated.     Dilution  may 
be  carried  to  such  a  point  that  ignition  will  not  take  place 
at  all.    As  the  dilution  increases  the  rise  in  pressure  during 
explosion  decreases  until  finally  there  is  practically  no  rise 
in  pressure.    Failure  to  ignite,  and  consequent  stopping  of 
the  engine,  may  result  from  either  too  rich  or  too  poor  a 
charge.    For  rapid  and  complete  combustion,  furthermore, 
the  mixture  of  air  and  fuel  should  be  as  intimate  as  possible. 

103.  INCOMPLETE  COMBUSTION. — Incomplete  combustion 
not  only  means  less  power,  and  a  waste  of  fuel,  but  may 
produce  carbon  deposits  in  the  cylinder.     Such   deposits 
are  liable  to  cause  p  re-ignition.     They  will  also  mix  with 
the  lubricating  oil  and  form  a  gummy  paste  which  rapidly 


EXPLOSIVE   MIXTURES 


75 


wears  the  cylinder,  piston,  and  valves.  It  is  chiefly  for 
this  latter  reason  that  the  heavy  oils  are  so  difficult  to 
handle  in  the  internal-combustion  engine.  Incomplete 
combustion  may  also  cause  explosions  in,  and  overheating 
of,  the  exhaust  pipe,  as  well  as  causing  a  bad  odor  and  a 
smoky  exhaust. 

104.  THE  COMBUSTION  CHAMBER  should  have  as  little 
cooling  surface  as  possible. 

There  should  be  no  projecting  points  or  corners.  These 
tend  to  become  incandescent  and  cause  pre-ignition. 


FIG.  29. 

There  should  be  no  corners,  pockets,  or  channels  in  which 
explosive  mixtures  may  lodge.  Under  certain  conditions 
explosions  in  such  pockets  and  channels  may  set  up  pulsa- 
tions, or  explosive  waves,  which  may  become  so  intense  as 
to  cause  serious  damage.  Fig.  29  shows  how  this  condition 
affects  the  indicator  card.  A  somewhat  similar  card  is  also 
produced  by  a  non-uniform  explosive  mixture. 

Since  burnt  gases  always  remain  in  the  cylinder  unless 
scavenged,  the  igniter  must  be  so  placed  that  the  fresh 
charge  can  reach  it.  Failure  to  ignite  will  obviously  result 
if  the  igniter  is  surrounded  by  burnt  gases. 


CHAPTER   X 


MIXING    VALVES   AND   CARBURETERS 

105.  MIXING  VALVES. — In  the  stationary  gas  engine  some 
provision  must  be  made  for  mixing  the  charge  of  air  and 
gas  thoroughly  before  it  enters  the  cylinder.  A  non- 
homogeneous  mixture  means  poor  combustion.  This  mix- 
ing is  usually  done  by  means  of  a  mixing  valve.  Fig.  30 
shows  a  simple  arrangement  for  this  purpose.  The  gas 
enters  at  A  and  its  flow  is  regulated  by  the  valve  B.  The 
air  enters  at  C  and  the  mixture  of  air  and  gas  passes  out 
at  D.  The  valve  B  can  be  arranged 
with  a  micrometer  attachment  for 
accurate  adjustment.  There  are,  of 
course,  different  styles  of  mixing 
valves,  but  the  object  in  each  is  to 
secure  an  intimate  mixture  of  the  air 
and  gas.  All  the  air  required  for 
combustion  may,  or  may  not,  pass 
through  the  mixing  valve. 

106.  VAPORIZERS.  —  In  stationary 
engines  using  a  liquid  fuel  a  vapor- 
izer is  used  for  atomizing  the  fuel  and 
mixing  it  with  the  air.     Fig.  31  shows 
the  general  arrangement  of  one  style 
of  vaporizer.     The  fuel  is  kept  under 
sufficient  pressure  to  carry  it  into  the  vaporizer  either  by 
elevating  the  tank   or    by  putting   the  liquid  under  air 
pressure.    A  small  pump  can,  of  course,  be  used  instead,  if 
76 


MIXING   VALVES  AND  CARBURETERS 


77 


desired.  The  fuel  enters  at  D  and  its  flow  is  regulated 
by  the  needle  valve  C.  During  the  suction  stroke  the 
air  enters  at  A  and  the 
fuel  is  atomized  in  pass- 
ing through  the  restricted 
opening  controlled  by  C1. 
The  overflow  pipe  E  leads 
away  the  surplus  fuel. 

107.  CARBURETERS.  — 
The  vaporizer  gives  very 
good  results  for  steady 
loads  and  speeds,  but  where 
the  speed  varies  greatly, 
and  changes  quickly,  a 
carbureter  is  used.  Fig.  32  illustrates  the  general  princi- 
ples upon  which  carbureters  are  constructed.  The  fuel 
enters  at  A  and  is  kept  at  a  constant  level  by  the  float  B. 


FIG.  32. 

The  float  is  so  adjusted  that  it  keeps  the  fuel  level  about 
TV"  below  the  opening  in  the  atomizer  C.    The  air  comes 


78  GAS-ENGINE   THEORY   AND   DESIGN 

in  through  D  and,  in  rushing  past  C,  creates  a  vacuum 
so  that  a  fine  atomized  spray  of  the  liquid  is  drawn 
from  C  and  mixed  with  the  air.  The  amount  of  car- 
bureted air  passing  through  E  can  be  regulated  by  the 
valve  F. 

Since  the  vaporizing  of  a  liquid  results  in  lowering 
temperature,  the  incoming  air  should  be  warmed  somewhat, 
and  this  can  be  done  by  flanging  the  pipe  D  and  leading 
some  of  the  hot  exhaust  gases  past.  The  atomizer  breaks 
the  liquid  up  into  very  fine  particles,  but  heating  the  fuel 
vaporizes  it  and  makes  a  more  intimate  mixture  of  air  and 
fuel  possible,  resulting  in  more  power  with  a  lower  fuel 
consumption. 

In  the  case  of  alcohol  and  kerosene  this  preheating  of 
air  is  necessary  in  order  to  obtain  the  best  results.  The 
air  may  be  heated  at  both  G  and  H.  When  kerosene  and 
the  heavier  oils  are  used  with  low  compression  the  oil  seems 
to  decompose  in  the  engine  cylinder  and  carbon  deposits 
result.  It  is  claimed  that  if  the  carbureted  charge  is  drawn 
through  a  vaporizer  at  a  red  heat  (H  in  Fig.  32)  that  no 
carbon  deposits  will  form  in  the  cylinder. 

Warming  the  incoming  air  accomplishes  three  purposes: 
it  restores  to  the  liquid  the  heat  lost  in  evaporation  and  so 
prevents  an  undue  cooling  of  the  liquid;  it  makes  a  more 
homogeneous  combustible  mixture,  which  means  more 
economy;  it  neutralizes  the  effect  of  moisture  in  the  air 
and  fuel. 

Too  much  moisture  interferes  with  combustion — al- 
though a  slight  amount  is  beneficial — and  trouble  is  some- 
times experienced  in  wet  weather  or  when  there  is  con- 
siderable water  in  the  fuel.  Gasolene,  kerosene  and  the 
other  oils  contain  more  or  less  water,  while  alcohol  is  usually 
largely  diluted  with  water. 

If  only  a  part  of  the  air  necessary  for  combustion  is  drawn 


MIXING   VALVES  AND   CARBURETERS  79 

through  the  carbureter  the  increase  in  volume  due  to 
heating  will  amount  to  very  little. 

The  starting  of  an  engine  operating  on  alcohol  or  kero- 
sene is  apt  to  be  more  difficult  than  in  the  case  of  gasolene 
unless  some  provision  is  made  for  warming  the  incoming 
air  previous  to  starting. 

There  is  a  considerable  difference  of  opinion  regarding 
the  value  of  pre-heating  as  discussed  above.  Some  ex- 
perimenters insist  that  better  results  are  obtained  by 
breaking  up  the  liquid  fuel  as  much  as  possible  by  means 
of  atomizers,  since,  if  the  preheated  charge  cools  on  its 
way  to  the  combustion  chamber,  the  fuel  will  simply  con- 
dense and  no  good  has  been  accomplished  by  pre-heating. 
Other  experimenters  claim  that  good  results  can  be  ob- 
tained by  pre-heating.  Experiments  made  by  the  author 
lead  him  to  believe  that  very  good  results  can  be  obtained 
by  pre-heating,  provided  care  is  taken  that  the  fuel  does  not 
condense  previous  to  ignition. 

The  cleanest  method  of  handling  any  fuel  in  the  com- 
bustion chamber  is  doubtless  by  introducing  it  in  the  form 
of  a  gas  free  from  impurities.  The  final  solution  of  the 
method  of  handling  heavy  oils  may  be  to  gasify  them.  One 
method  of  doing  this  has  been  mentioned  under  "Oil 
Water  Gas." 

A  point  that  must  be  borne  in  mind  in  carbureter  design 
is  that  while  the  mixture  may  be  right  for  slow  speeds,  it 
will  be  too  rich  at  high  speeds,  or,  if  right  for  high  speeds, 
it  may  be  too  lean  for  slow  speeds.  Provision  must  there- 
fore be  made  to  secure  the  right  mixture  at  varying  speeds. 

Another  point  is  that  the  air  should  pass  through  D  at 
a  fairly  high  speed  in  order  to  obtain  the  best  atomizing 
effect.  A  speed  of  70  to  80  feet  per  second  will  give  good 
results. 

A  carbureter  must  be  so  designed  that  it  will  work 


80  GAS-ENGINE  THEORY  AND   DESIGN 

properly  in  spite  of  the  jolting  it  may  receive  in  an  auto- 
mobile or  boat,  and  when  the  engine  is  tipped  up  and  down 
at  different  angles. 

The  fuel  must  be  thoroughly  filtered  before  it  roaches 
the  carbureter. 

Students  arc  sometimes  at  a  loss  to  understand  how  a 
slight  variation  in  the  adjustment  of  the  mixing  valve  or 
carbureter  may  make  a  large  difference  in  the  horse-power. 
If  we  take  the  calorific  power  of  gasolene  to  be  18,000 
B.T.U.  per  pound,  then  this  is  equivalent  to  14,004,000 
ft.-lbs.  If  a  four-cycle  engine  running  at  1,000  R.P.M.  con- 
sumes 1  pint  of  gasolene  per  H.-P.  hour,  then  the  oil  con- 
sumption per  power  stroke  is  1/240,000  gallon  per  H.-P. 


CHAPTER  XI 

GOVERNING 

108.  FUNCTIONS  OF  THE  GOVERNOR. — An  engine  slows 
down  as  the  load  increases,  and  runs  faster  as  the  load  de- 
creases; therefore  a  governor  is  necessary  to  take  care  of 
the  variation  of  load  by  varying  the  power  when  the  engine 
is  desired  to  run  at  a  constant  speed — as  is  the  case  in 
stationary  practice.    The  governor  may,  of  course,  be  ar- 
ranged so  that  it  can  be  set  for  different  speeds.    Where 
both  the  speed  and  the  load  vary,  as  in  automobile  prac- 
tice, a  wide  range  of  speed  and  power  can  be  obtained  by 
throttling  the  charge,  and  by  also  advancing  and  retarding 
the  ignition. 

Reliability  and  economy  under  varying  loads  are  mat- 
ters of  prime  importance,  and  must  be  given  careful  con- 
sideration. 

A  brief  outline  of  the  principal  systems  of  governing 
will  be  given. 

109.  IMPOVERISHING  THE  CHARGE. — Under  this  system 
the  quantity  of  fuel  used  per  power  stroke  is  diminished  in 
order  to   diminish  the   power.     The  advantage   of  this 
method  is  that  the  compression  always  remains  the  same, 
and  therefore  the  highest  efficiency — so  far  as  compression 
is  concerned — is  maintained.    The  disadvantages  are  that 
the  fuel  must  heat  up  an  excess  of  air  when  running  under 
light  loads  and  the  mixture  may  become  so  weak  that  it 
will  not  ignite  at  all  in  the  cylinder,  in  which  case  fuel 
is  wasted  and  may  burn  in  the  exhaust  pipe.    This  method 

6  81 


S2 


GAS-ENGINE   THEORY   AND   DESIGN 


is  also  called  quality  governing,  since  the  quality  of  the 
mixture  is  changed. 

110.  THROTTLING  THE  CHARGE. — Under  this  system  the 
governor  acts  on  a  valve  which  is  so  arranged  that  the 
charge  of  air  and  fuel  is  increased  or  diminished  as  the  load 
increases  or  decreases.  The  proportions  of  air  and  fuel  are 
not  changed,  but  only  the  amount  drawn  into  the  cylinder. 
This  method  is  used  in  the  Westinghouse  and  a  number 
of  other  stationary  engines,  as  well  as  in  the  majority  of 
automobile  engines  where  the  throttling  is  done  by  hand. 


The  disadvantage  of  this  method  lies  in  the  fact  that  the 
compression  varies  with  the  volume  of  the  charge  drawn 
in  with  a  consequent  decrease  in  economy.  The  economy 
is  always  less  for  light  and  overloads  than  for  full  load,  but 
the  difference  in  economy  is  frequently  much  more  than  it 
should  be.  Throttling  the  charge  will  usually  allow  the 
engine  to  run  under  lighter  loads  than  impoverishing  the 
charge.  This  second  method  is  also  called  quantity  govern- 
ing, since  the  quantity  of  explosive  mixture  is  changed. 

Since  the  rate  of  combustion  varies  with  the  compression 
the  ignition  point  should  be  advanced  for  light  loads, 
therefore  the  ignition  point  as  well  as  the  throttle  should 


GOVERNING  83 

be  controlled  by  the  governor.    This  idea  is  carried  out  in 
the  Rathbun  and  some  other  stationary  engines. 

Fig.  33  shows  the  effect  on  the  indicator  card  of  varying 
the  load  in  a  throttling  engine. 

111.  GOVERNING  BY  CUT-OFF. — In  this  system  the  charge 
is  admitted  during  a  part  of  the  suction  stroke  and  then 
cut  off  by  closing  the  valve.    It  is  the  same  as  throttling 
in  so  far  that  an  incomplete  charge  is  drawn  into  the 
cylinder  at  less  than  full  load,  but  there  is  no  wire-drawing, 
since  there  is  no  throttling. 

When  governing  by  throttling  or  cut-off,  provision  should 
be  made  to  prevent  opening  of  valves  due  to  the  vacuum 
in  the  cylinder. 

Combinations  of  quality  and  quantity  governing  have 
been  tried,  so  as  to  combine  the  good  points  of  each,  and 
are  in  use  in  several  makes  of  engines. 

112.  ADVANCING  AND  RETARDING  THE  SPARK. — When 
ignition  takes  place  before  the  piston  has  completed  its 
compression  stroke,  the  spark  is  said  to  be  advanced; 
when  the  spark  is  set  past  the  dead  center  in  the  other 
direction  so  that  ignition  takes  place  when  the  expansion 
stroke  has  already  begun,  the  spark  is  said  to  be  retarded. 
Retarding  the  spark  results  in  a  late  explosion — diminish- 
ing the  effective  pressure  on  the  piston,  but  also  wasting 
fuel.    Advancing  the  spark  a  little  means  earlier  ignition 
and  more  complete  combustion,  and  is  of  considerable  im- 
portance in  high-speed  engines,  but  the  spark  may  be  ad- 
vanced so  far  that  it  will  cause  a  back  explosion  and 
diminish  the  power  of  the  engine — if  nothing  worse. 

The  spark  may  be  advanced  and  retarded  in  connection 
with  the  throttling  and  cut-off  methods  of  governing. 

Any  good  style  of  governor  can  be  used.  The  governor 
is  connected  so  as  to  operate  the  throttle  valves  and  timer 
in  whatever  manner  may  be  most  convenient. 


CHAPTER  XII 

IGNITION 

113.  Ignition  may  be  brought  about  by  means  of  a  hot 
chamber,  high  compression,  or  the  electric  spark. 

114.  HOT-CHAMBER  IGNITION. — This  is  one  of  the  earliest 
forms  of  ignition  and  is  still  used  in  some  gas  and  oil  engines. 
The  arrangement  for  an  oil  engine  is  shown  in  Fig.  34. 
A  cast-iron  chamber,  A,  opens  into  the  cylinder.    Previous 
to  starting,  this  chamber  is  heated  by  means  of  a  torch  to 
a  dull-red  heat  and  when  the  engine  is  running  the  oil  is 

sprayed  either  into  the  chamber  direct, 
or  into  the  cylinder,  and  is  then  carried 
into  the  chamber  during  the  com- 
pression stroke.  The  point  at  which 
ignition  takes  place  depends  upon  the 
time  at  which  the  oil  is  injected  and 
upon  the  general  form  of  the  hot 
FIG.  34.  chamber.  The  narrow  neck  connecting 

the  chamber  with  the  cylinder  delays 
combustion.  When  the  engine  has  been  running  a  little 
while  the  torch  may  be  taken  away,  as  the  chamber  is  now 
kept  at  the  proper  temperature  by  the  heat  of  combustion. 
The  disadvantages  of  the  above  method  are  the  time 
required  for  starting  and  the  impossibility  of  timing  the 
ignition  with  anything  like  accuracy,  especially  with  a  fuel 
of  varying  quality.  The  advantages  are  simplicity  and  that 
the  hot  chamber  contains  sufficient  heat  to  vaporize  and 
ignite  the  heavy  oils. 

84 


IGNITION 


So 


In  the  gas  engine  a  small  porcelain  tube  opening 
the  cylinder  is  used  in  place  of  the  cast-iron  chamber. 

115.  IGNITION    BY    COMPRESSION. — Air   may    be   com- 
pressed to  such  a  degree  that  the  temperature  will  be  1000°, 
or  even  more — a  temperature  amply  sufficient  to  ignite  any 
fuel.    Where  such  high  compression  is  used  the  fuel  must 
be  injected  when  the  compression  stroke  has  been  com- 
pleted, otherwise  pre-ignition  takes  place. 

Note. — See  also  Cooling  by  Water  Injection. 

116.  ELECTRIC  IGNITION. — Electric  ignition  is  now  in 
almost  universal  use  on  gas  and  gasolene  engines.     The 
great  advantage  of  this  method  is  that  ignition  can  be  timed 
with  absolute  certainty.    The  principal  systems  of  electric 
ignition  will  be  briefly  described. 

The  source  of  the  electric  current  may  be  a  primary 
battery,  a  secondary  (stor- 
age) battery,  a  dynamo  or  a 
magneto. 

A  dynamo  is  self-exciting, 
while  the  field  of  the  mag- 
neto is  composed  of  per- 
manent steel  magnets. 

117.  THE    JUMP- SPARK 
SYSTEM. — Fig.  35  is  a  dia- 
gram of  the  jump-spark  sys- 
tem.     This  is  made  up  of 
a   battery   (usually   several 
primary  batteries  connected 
in  series)  A  which  furnishes 


FIG.  35. 


the  current,  a  revolving  disc  B  which  opens  and  closes  the 
circuit  (the  shaded  portion  of  the  disc  is  a  conductor,  the 
rest  is  a  non-conductor),  an  induction  coil  C  and  a  spark- 
plug D  which  is  screwed  into  the  engine  cylinder. 
In  the  spark-plug,  as  here  shown,  are  two  insulated  wires 


86 


GAS-ENGINE  THEORY   AND   DESIGN 


which  project  into  the  cylinder  and  have  the  ends  so  bent 
that  the  current  has  to  jump  across  a  small  air  space.  In 
jumping  across  this  space  the  current  produces  a  spark 
which  ignites  the  explosive  mixture. 

118.  THE  INDUCTION  COIL. — The  function  of  the  induc- 
tion coil  is  to  convert  the  low-tension  battery  current  into 
a  high-tension  current  which  is  capable  of  jumping  across 
the  air  gap  between  the   plug  terminals.     A  low-tension 
current  cannot  do  this.    The  coil  is  composed  of  an  iron 
core  a,  a  primary  winding  of  a  few  turns  of  heavy  wire  6, 
a  secondary  winding  of  many  turns  of  fine  wire  c,  a  small 
piece  of  iron  d  held  against  an  adjustable  screw  e  by  a 
spring,  and  a  condenser  /.     When  the  primary  circuit  is 
closed  by  B  a  low-tension  direct  current  flows  through  the 
primary  winding  and  magnetizes  a  which  attracts  d.    As 
d  jumps  toward  a  it  breaks  the  circuit,  the  current  ceases 

to  flow  and  a  ceases  to  be  a 
magnet,  so  the  spring  pulls  d 
back  against  e.  This  breaking 
and  closing  of  the  circuit  at  d 
takes  place  very  rapidly  and  con- 
tinues so  long  as  the  circuit  is 
not  broken  at  B.  d  is  called  a 
vibrator.  The  action  of  the  pri- 
mary current  induces  a  high 
tension  alternating  current  in  C. 
The  condenser  /  consists  of  sev- 
eral sheets  of  tinfoil  insulated 
from  each  other  and  its  function 
is  to  store  the  current  at  one  period  and  give  it  out  at 
another,  increasing  the  efficiency  of  the  coil  and  preventing 
injurious  sparking  at  d.  This  system  is  used  principally 
on  high-speed  engines. 

119.  THE  MAKE-AND-BREAK  SYSTEM. — Fig.  36  is  a  dia- 


FIG.  36. 


IGNITION 


,S7 


gram  of  the  make-and-break  system.  The  current  is 
furnished  by  the  battery  A,  the  cam  B  pushes  the  rod  D 
into  the  cylinder  so  that  it  makes  a  sliding  contact  with  E. 
The  object  of  a  sliding  contact  is  to  remove  any  soot  which 
may  be  on  the  contact  surfaces.  The  soot  would  act  as  an 
insulator  and  prevent  the  current  from  passing.  The  cir- 
cuit is  completed  by  a  primary  winding  around  the  iron 
core  C.  When  the  cam  releases  D  the  latter  is  pushed  back 
by  a  spring,  breaking  the  contact  with  E,  and  as  the  con- 


FIG.  37. 

tact  is  broken  a  spark  is  produced.  The  current  here  used 
is  a  low-tension  direct. 

The  ordinary  dynamo  or  magneto  will  not  furnish  suf- 
ficient current  at  a  slow  speed  for  a  good  spark,  and  for 
this  reason  engines  are  frequently  started  on  a  battery 
current  and  after  they  have  speeded  up  the  battery  is  cut 
out  and  the  current  is  furnished  by  a  magneto.  In  Fig.  36, 
a  and  b  are  switches,  and  F  is  a  magneto,  or  dynamo, 
driven  by  the  engine. 

The  make-and-break  system  is  generally  used  on  slow- 
and  medium-speed  engines.  It  is  more  reliable  and  fur- 
nishes a  better  spark  than  the  first  system,  but  has  the  dis- 


GAS-ENGINE  THEORY   AND   DESIGN 


advantage  of  requiring  moving  parts  in  the  engine  cylinder. 
This  system  is  also  used  on  alcohol  and  kerosene  engines 
since  these  fuels  are  more  difficult  to  ignite  than  gasolene 
or  gas,  and  require  a  better  spark  than  the  jump-spark 
generally  furnishes. 

120.  TIMERS,  DISTRIBUTORS. — Fig.  37  shows  the  wiring, 
etc.,  for  a  multiple-cylinder  engine.    A  is  the  magneto,  B 
is  the  primary  winding  of  the  induction  coil,  D  is  the  timer, 
E  is  the  distributor,  C  is  the  secondary  winding,  1,  2,  3,  and 
4  are  four  cylinder-heads  into  which  the  sparking-plugs  are 
screwed.     In  the  position  shown  the  current  is  flowing 
through  the  primary  circuit.    In  the  secondary  circuit  the 
plug  in  cylinder  1  is  receiving  current.    The  discs  D  and  E 

are  mounted  together 
on  one  shaft  and  it  can 
readily  be  seen  how  all 
four  plugs  receive  cur- 
rent successively  during 
one  revolution  of  the  dis- 
tributor shaft.  The  cyl- 
inders in  this  case  are 
usually  fired  in  the  order 
1,  3,  4,  2.  Some  six- 
cylinder  engines  are  fired 
in  the  order  1,4, 2, 6, 3, 5. 
The  ignition  can  be 
advanced  or  retarded 
by  rotating  the  casing  carrying  the  brushes  aa  and  contact- 
points  bbbb. 

In  all  electric-ignition  systems  proper  insulation  must, 
of  course,  be  provided  for. 

121.  STORAGE-BATTERY  SYSTEM. — In  Fig.  38  (the  Apple 
system)  the  dynamo  A  charges  the  storage  battery  B,  and 
the  current  required  for  ignition  is  taken  from  the  battery; 


FIG.  38 


IGNITION 


89 


dddd  are  induction  coils  and  eeee  the  spark-plugs.  These 
plugs  may  be  grounded  on  the  frame  so  that  only  one  re- 
turn wire  is  necessary.  This  does  away  with  the  wiring 
enclosed  by  the  dotted  lines. 

It  will  be  noticed  that  the  double-disc  system  in  Fig.  37 
does  away  with  the  use  of  a  separate  coil  for  each  spark- 
ing-plug. 

The  advantages  of  the  dynamo  and  storage- battery  com- 
bination are  that  current  is  always  available  for  both 
ignition  and  lighting,  and  starting  can  be  done  without 
having  a  primary  battery  in  circuit. 


FIG.  39. 

Fig.  39  shows  a  very  simple  ignition  system  (Atwater 
Kent).  This  consists  of  a  primary  battery,  induction  coil, 
distributor,  and  sparking  plugs.  Since  the  amount  of  cur- 
rent required  for  ignition  is  very  small,  and  there  is  no  waste 
of  current,  it  is  claimed  that  the  battery  will  last  a  long  time. 

122.  HIGH-  AND  LOW-TENSION  MAGNETOS.— Magnetos  are 
now  on  the  market  which  are  so  wound  that  no  induction 
coil  is  required,  and  which  will  furnish  sufficient  current 
at  a  slow  speed  for  starting,  so  no  primary  battery  is  re- 
quired. A  prominent  one  is  the  Bosch. 

In  the  Bosch  high-tension  magneto  both  the  primary 
and  secondary  windings  are  on  the  armature  so  that  the 
magneto  furnishes  an  alternating  high-tension  current 


90  GAS-ENGINE  THEORY   AND   DESIGN 

direct.  The  magneto  also  contains  a  distributor  and  pro- 
vision for  advancing  and  retarding  the  spark.  The  arma- 
ture rotates  and  the  speed  of  rotation  depends  upon  the 
number  of  engine  cylinders  supplied  with  current. 

The  Bosch  low-tension  magneto  furnishes  an  alternating 
current.  The  armature  oscillates  instead  of  rotating.  The 
low-tension  current  may  be  used  with  a  make-and-brcak 
mechanism,  or  with  a  special  spark-plug  which  is  mag- 
netically operated.  The  magnetic  plug  permits  a  low- 
tension  current  to  be  used  for  high-speed  work,  with 
consequently  no  insulation  troubles.  No  mechanical 
make-and-brcak  mechanism  is  required.  The  plugs  are 
connected  to  the  magneto  with  single-wire  cables. 

123.  CONCLUSION. — In  conclusion  it  may  be  stated*  that 
a  weak  spark  will  not  ignite  an  explosive  mixture:  there 
must  be  sufficient  heat  to  start  combustion;  a  high  tem- 
perature of  the  spark  is  not  enough  if  the  spark  is  of 
very  short  duration.  A  poor  ignition  system  will  not 
furnish  a  good  spark. 

The  ignition  should  be  arranged  to  take  place  near  the 
centre  of  the  explosive  mixture. 

If  the  points  of  a  spark-plug  become  coated  with  soot 
the  spark  cannot  jump  across.  Engines  are  sometimes  ar- 
ranged with  two  plugs  in  each  cylinder  so  that  if  one  set 
causes  trouble  the  other  set  can  be  used,  thus  avoiding 
stoppage  of  the  engine,  or  both  sets  can  be  used  together. 

The  insulation  should  be  protected  from  heat,  oil,  water, 
etc.,  and  all  connections  should  be  so  arranged  that  they 
cannot  work  loose. 


CHAPTER  XIII 


COOLING 

124.  In  order  to  prevent  overheating  of  the  cylinder  walls, 
cylinder  head,  valves,  etc.,  it  is  necessary  to  make  provision 
for  carrying  away  a  part  of  the  heat  generated  during  com- 
bustion.   This  is  done  by  using  either  water  or  air  as  the 
cooling  medium. 

125.  WATER  COOLING. — On  account  of  its  great  heat- 
absorbing  capacity  water  is  generally  used.    Fig.  40  shows 
the  siphon  system  as  applied  to  a  horizontal  engine.    The 


1 


FIG.  40. 


FIG.  41. 


cold  water  enters  the  water  jacket  from  below  and  at  the 
hottest  end  of  the  cylinder.  The  hot  water  rises  to  the  top 
and  establishes  a  circulation. 

When  a  more  rapid  circulation  of  the  water  is  necessary 
a  pump  is  used. 

Fig.  41  illustrates  the  method  of  cooling  used  for  auto- 
mobile engines.  Since  a  large  water-tank  cannot  be  carried 
a  special  arrangement  must  be  provided  for  getting  rid  of 
91 


92  GAS-ENGINE   THEORY   AND   DESIGN 

the  surplus  heat.  The  pump  A  is  geared  to  the  crank  or 
cam  shaft.  The  radiator  B  is  of  the  usual  honeycomb  style, 
having  a  very  large  radiating  surface.  The  fan  C  draws  air 
through  the  openings  in  the  radiator  so  that  a  large  amount 
of  heat  can  be  abstracted  in  a  short  time.  This,  as  most 
water-cooling  systems,  is  really  a  combination  of  water  and 
air  cooling. 

At  slow  speeds,  and  where  the  water  is  free  from  injurious 
substances,  a  plunger  pump  may  be  used.  In  marine  prac- 
tice, where  the  cooling  fluid  is  taken  from  the  water  in 
which  the  boat  moves,  a  centrifugal  pump  is  generally  best, 
since  it  will  allow  dirt  and  small  obstacles  to  pass  through 
without  becoming  clogged.  The  inlet  end  of  the  water 
pipe  should  be  protected  by  fine  wire  gauze  so  that  only 
very  small  obstacles  can  pass  through.  The  rapid  and  free 
circulation  of  the  water  is  usually  counted  upon  to  cany 
the  impurities  out  again. 

A  point  to  be  borne  in  mind  is  that,  while  a  pump  may 
furnish  sufficient  water  at  a  high  speed,  it  may  not  furnish 
enough  to  prevent  the  engine  from  overheating  when  run- 
ning at  a  slow  speed.  If  the  pump  furnishes  sufficient  water 
at  a  slow  speed  it  may  cool  the  engine  too  much  at  a  high 
speed. 

126.  COOLING  BY  BOILING. — A  good  method,  where  there 
is  no  scarcity  of  water,  is  to  simply  allow  the  water  to  boil 
away.    This  keeps  the  entire  water  jacket  at  a  temperature 
of  about  212°  and  the  temperature  of  the  water  remains 
the  same,  no  matter  whether  the  engine  is  running  fast  or 
slow.    The  amount  of  water  boiled  away  in  this  manner 
per  H.-P.  will  be  quite  small.    This  does  away  with  a  large 
cooling-tank,  pump,  piping,  etc.     The  steam  can  also  be 
led  into  the  exhaust  passage  and  so  help  to  cool  it. 

127.  Am  COOLING. — In  order  to  do  away  with  a  large 
radiator,  pump,  piping,  joints  which  may  become  leaky, 


COOLING  93 

etc.,  several  automobile  manufacturers  have  adopted  the 
air-cooling  system  in  which,  Fig.  42,  the  engine  cylinder 
is  provided  with  a  number  of  flanges,  giving  a  large  radiat- 
ing surface,  and  the  cooling  effect  is  further  increased  by 
forced-air  circulation.  The  fuel  economy  is  somewhat  higher 
than  in  the  water-cooled  systems,  since  the  temperature  of 
the  cylinder  walls  is  higher.  The  cylinder  requires  more 
lubricating  oil  than  a  water-cooled 
cylinder.  This  system  is  not  appli- 
cable for  cylinders  of  more  than  10 
H.-P.  since  the  heat  cannot  be  carried 
off  with  sufficient  rapidity. 

In  this  connection  it  might  be  men- 
tioned  that   cooling   becomes  a  very  FIQ  42 
serious  problem  as  the  cylinder  dimen- 
sions increase.    This  is  especially  so  in  the  case  of  high-speed 
engines  whose  limit  at  present  seems  to  be  7  x  1"  or  7  x  8". 

128.  COOLING  BY  WATER  INJECTION. — Many  experiments 
have  been  made  with  water  injection  with  a  view  to  lower- 
ing the  water-jacket  losses,  the  idea  being  that  the  water 
injected  into  the  combustion  chamber  would  absorb  a  part 
of  the  surplus  heat  of  combustion  and  by  its  expansion  (as 
steam)  increase  the  power  of  the  engine.  The  result  has 
been  that  the  temperature  was  lowered  too  much  and  a 
loss  instead  of  a  gain  resulted.  In  the  Banki  motor  water 
is  used  for  the  purpose  of  absorbing  a  part  of  the  heat 
generated  during  compression  and  so  decrease  the  work  of 
compression,  also  making  higher  compression  pressures 
possible  without  danger  of  pre-ignition.  In  this  engine  a 
water- vaporizer  is  located  in  the  air-suction  pipe,  and  fuel, 
water,  and  air  are  drawn  into  the  cylinder  together.  The 
compression  is  about  230  Ibs.,  and  the  explosion  pressure 
is  about  600  Ibs.  The  thermal  efficiency  is  about  30  per 
cent. 


94  GAS-ENGINE  THEORY   AND   DESIGN 

129.  COOLING  WATER  REQUIRED. — The  amount  of  water 
required  for  cooling  may  be  calculated  by  figuring  that  the 
heat  carried  away  by  the  jacket  water  is  equal  to  the  in- 
dicated horse-power,  and  then  taking  the  difference  between 
the  temperatures  of  the   incoming  and   outgoing   water. 
The  latter  should  be  in  the  neighborhood  of  180°  and 
should  be  kept  as  nearly  constant  as  possible. 

The  tank  capacity  for  stationary  engines  is  usually  fig- 
ured as  25  gal.  per  H.-P. 

In  automobile  engines  about  1  gal.  per  H.-P.  is  figured, 
and  the  radiator  must  have  sufficient  capacity  to  dispose  of 
the  total  B.T.U.  as  rapidly  as  they  are  absorbed  by  the 
water. 

In  air  cooling  the  amount  of  radiating  surface  required 
varies  with  the  design.  In  the  Franklin  engine  an  auxiliary 
exhaust  valve  is  used  for  disposing  of  the  exhaust  gases 
quickly. 

The  auxiliary  exhaust,  which  is  described  elsewhere, 
has  an  important  bearing  on  the  cooling  of  the  cylinder. 

130.  ANTI-FREEZING  SOLUTIONS. — In  order  to  prevent 
the  jacket  water  from  freezing  during  cold  weather,  while 
the  engine  is  not  running,  various  substances  are  added 
to  the  water.    Among  these  are  glycerine  in  proportions  of 
half  and  half;  also  calcium  chloride  in  proportions  of  1  to  2 
by  weight.    The  filtered  solutions  should  be  used.    Alcohol 
is  sometimes  used  in  place  of  water  and  this  will  freeze  at 
a  lower  temperature  than  either  of  the  above.    Oil  has  also 
been  used  in  place  of  water. 

Draining  off  the  jacket  water  when  the  engine  is  not  in  use 
will  overcome  this  trouble  in  the  case  of  a  stationary  engine. 

Water  containing  lime,  or  any  substance  that  will  either 
form  a  coating  or  corrode  the  metal,  must  not  be  used.  If 
this  point  is  neglected,  a  reliable  running  of  the  engine 
cannot  be  expected. 


A.EE-X 


CHAPTER  XIV 

EXHAUST 

131.  The  noise  of  the  exhaust  should  be  muffled  as  much 
as  possible.  This  is  caused  by  the  rapid  expansion  of  the 
exhaust  gases  when  they  strike  the  atmosphere.  In  sta- 
tionary installations  the  gases  should  be  carried  away  in 
such  a  manner  that  they  will  not  cause  annoyance  on  ac- 
count of  noise,  odor,  or  smoke.  The  exhaust  should  be 
arranged  so  that  no  damage  will  result  from  overheating  of, 
or  explosions  in,  the  exhaust  pipe.  The  muffling  should  be 
done  without  causing  back  press- 
ure. The  engine  exhaust  pas- 
sage is  sometimes  water-jacketed 
to  prevent  overheating.  Fig.  43 
shows  the  customary  method 
of  arranging  the  exhaust  in  sta- 
tionary engines.  The  exhaust 
gases  from  the  engine  pass 
through  the  pipe  A  into  a  cast- 
iron,  or  riveted  steel,  vessel,  B, 
which  is  placed  some  distance 
underground.  The  gases  ex- 
pand and  cool  to  some  extent 
in  B,  and  then  pass  out  and 
into  the  atmosphere  through 
the  pipe  C.  By  making  B  fairly  large  in  proportion  to  the 
cylinder  volume  (at  least  ten  times  this  volume) ,  and  the 
pipe  C  long,  a  practically  noiseless  exhaust  is  obtained. 
95 


FIG.  43. 


96  GAS-ENGINE  THEORY   AND   DESIGN 

132.  THE  MUFFLER  for  portable  engines  should  be  so 
designed  as  to  secure  a  gradual  expansion  of  the  gases  and 
consequent  reduction  of  pressure,  the  speed  of  the  exhaust 
gases  depending  upon  the  diameter  of  the  exhaust  passage, 


,-      ^    A     x 

\         e=^  .^  ^  1  ^ 

t 

^      1       -     | 

FIG.  44. 

which  should  be  as  large  as  convenient.  Fig.  44  shows  the 
arrangement  of  a  muffler  which  may  be  placed  near  the 
engine.  The  sketch  is  self-explanatory. 

133.  A  MARINE  EXHAUST  is  shown  in  Fig.  45.  The  ex- 
haust gases  are  discharged  through  the  bottom  of  the  boat, 
and  consequently  under  the  water,  doing  away  with  noise 
and  smoke.  A  valve  should  be  provided  so  that  the  ex- 
haust passage  can  be  closed  when  desired ;  otherwise  if  the 
engine  sets  low  in  the  boat,  the  water  may  back  up  into 
the  cylinders  when  the  engine  is  not  running.  The  cooling 


FIG.  45. 

and  consequent  reduction  of  volume  of  the  exhaust  gases 
is  further  assisted  by  leading  the  discharging  cooling-water 
into  the  exhaust  passage  through  pipe  A  as  shown.  In  the 
under-water  exhaust  care  must  be  taken  to  proportion  the 
passages  so  that  there  will  be  as  little  back  pressure  as 
possible.  This  is  especially  the  case  in  the  small  two-cycle 
engine,  which  is  usually  so  sensitive  to  back  pressure  that 
the  engine  may  easily  bo  stopped  from  that  cause. 


EXHAUST  97 

134.  THE  AUXILIARY  EXHAUST. — In  a  number  of  engines 
an  auxiliary  exhaust  valve  operated  by  a  cam,  or  an  auxil- 
iary exhaust  port  uncovered  by  the  piston  toward  the  end 
of  the  stroke,  is  provided  for  several  reasons.  As  the  gases 
f)ass  through  the  auxiliary  exhaust  the  pressure  in  the 
cylinder  falls  rapidly  and  the  exhaust  valve  proper  is  not 
forced  open  against  a  considerable  pressure,  as  is  ordinarily 
the  case.  The  hottest  gases  pass  out  through  the  auxiliary 
port,  lessening  the  danger  of  overheating  the  exhaust  valve. 
The  exhaust  gases  are  disposed  of  quicker  and  a  cooler 
cylinder  results.  In  the  larger  stationary  engines  a  bal- 
anced water-cooled  exhaust  valve  dispenses  with  the 
necessity  of  an  auxiliary  exhaust. 


CHAPTER  XV 

SELECTION    OF  TYPE 

135.  In  selecting  the  type  of  engine  to  be  designed  the 
advantages  and  disadvantages  of  the  various  constructions 
must  be  studied  and  the  designer  can  then  choose  the  type 
most  suitable  for  the  work  to  be  done.    Following  is  the 
general  classification  according  to  the  mechanical  construc- 
tion : 

Two-cycle  or  four-cycle. 
Horizontal  or  vertical. 
Single-acting  or  double-acting. 
Single-cylinder  or  multiple-cylinder. 

136.  TWO-CYCLE  OR  FOUR-CYCLE. — Early  experiments 
with  two-cycle  engines  proved  unsatisfactory  and  the  four- 
cycle type  was  built  almost  exclusively  for  a  while.    During 
the  past  few  years  the  two-cycle  type,  in  both  small  and 
large  horse-powers,  especially  in  the  large,  has  proven  very 
successful.     In  small  powers  the  two-cycle  type  has  the 
advantage  of  simplicity  and  cheapness  and  is  at  the  present 
time  very  largely  used  in  marine  practice.    In  large  powers 
the  two-cycle  type  is  almost  a  necessity  since  the  four-cycle 
machine  becomes  excessively  bulky  for  the  power  developed. 
Many  of  the  objections  which  apply  to  the  small  two-cycle 
engine  with  crank-case  compression  do  not  apply  to  the 
large  machines,  where  a  charge  of  air  and  gas  is  delivered 
to  the  engine  cylinder  by  means  of  separate  pumps.    The 
advantages  of  this  latter  type  are  that  the  cylinder  is  com- 
pletely charged  with  a  combustible  mixture  and  that  there 

96 


SELECTION   OF  TYPE  99 

is  a  power  stroke  for  each  revolution.  The  disadvantages 
are  that  the  time  during  which  a  fresh  charge  can  be  ad- 
mitted, and  the  burnt  gases  exhausted,  is  exceedingly  short, 
and  that  there  is  the  extra  pump  mechanism.  In  the  small 
engine  with  crank-case  compression  the  volume  of  the  fresh 
charge  is  somewhat  less  than  the  piston  displacement  with 
a  consequently  smaller  amount  of  power  developed.  There 
is  also  more  or  less  danger  from  back  explosions  since  the 
hot  gases  and  fresh  charge  come  in  contact. 

The  advantages  of  the  four-cycle  engine  are  chiefly  the 
greater  length  of  time  available  for  exhausting  the  cylinder 
and  for  filling  it  with  a  fresh  charge.  The  disadvantages 
are  that  there  is  only  one  power  stroke  for  every  two  revo- 
lutions, and  the  extra  valve  gearing.  The  claim  generally 
made  for  the  four-cycle  engine,  that  it  is  more  reliable  than 
the  two-cycle,  unfortunately  is  often  true,  but  does  not  hold 
good  for  a  properly-designed  two-cycle  machine. 

In  the  small  two-cycle  engine  the  work  done  in  com- 
pressing the  charge  in  the  crank-case,  and  in  the  large  engine 
the  work  done  in  operating  the  pumps,  must  be  deducted 
from  the  I.H.P. 

137.  HORIZONTAL  OR  VERTICAL. — For  stationary  gas 
engines  of  large  power  (especially  double-acting  engines)  the 
horizontal  form  is  used  almost  exclusively  at  the  present 
time,  but  the  vertical  type,  on  account  of  its  many  inherent 
advantages  (especially  in  the  multiple-cylinder  form)  is 
steadily  gaining  in  favor. 

The  horizontal  type  is  heavier  and  bulkier,  the  cylinder 
is  more  apt  to  spring  out  of  shape,  the  weight  of  the  recip- 
rocating parts  causes  extra  wear  on  the  lower  side  of  the 
cylinder,  and  it  is  more  difficult  to  make  the  piston  air-tight. 
The  advantages  are  that  the  water-cooling  of  the  various 
parts  is  generally  more  readily  accomplished,  the  impurities 
(carbon,  etc.)  are  swept  out  with  the  exhaust — a  very  im- 


100  GAS-ENGINE  THEORY   AND   DESIGN 

portant  point,  and  in  the  double-acting  two-cycle  type  the 
arrangement  of  the  valves  is  simpler. 

The  vertical  type  is  lighter,  more  compact,  lends  itself 
to  multiple-cylinder  construction,  possesses  a  greater 
mechanical  efficiency,  the  lubrication  is  better,  the  arrange- 
ment of  valve  gearing  is  simpler,  it  is  capable  of  better 
balancing  and  higher  speed,  the  various  parts  can  be  made 
more  accessible,  it  is  cheaper  to  manufacture  and  install. 
It  would  seem  as  if  a  vertical,  double-acting,  multiple- 
cylinder  type  would  possess  many  advantages  over  the 
horizontal  twin-engine  construction — the  opinions  of  some 
experts  to  the  contrary  notwithstanding. 

138.  SINGLE- ACTING  OR  DOUBLE-ACTING. — The  single- 
acting  engine  is  simple  and  easy  of  construction  in  small 
powers.  In  large  powers,  however,  it  becomes  excessively 
bulky,  the  various  parts  become  heavy  and  difficult  to 
manufacture,  sound  castings,  accurate  machine  work,  and 
resulting  reliability  of  performance  are  difficult  to  obtain. 
When  the  reciprocating  and  rotating  parts  weigh  many 
tons  they  are  expensive  .to  manufacture. 

The  double-acting  cylinder  is  much  smaller  for  the  same 
'power,  or,  for  the  same  size,  there  is  double  the  power  with 
a  moderate  increase  in  length.  It  has  more  mechanism. 
Since  there  is  a  crosshead  there  is  no  side  thrust  on  the 
piston.  The  double-acting  cylinder  requires  a  better 
cooling  arrangement  for  valves,  piston,  etc.,  which  must  be 
water-cooled  as  well  as  the  cylinder.  This  type  becomes  a 
necessity  for  large  powers. 

1.39.  SINGLE-CYLINDER  OR  MULTIPLE-CYLINDER. — For  a 
given  power  the  single-acting  cylinder  is  bulkier,  heavier, 
irregular  in  running,  and,  except  for  small  powers,  more 
expensive.  There  is  a  greater  pressure  on  the  cylinder-head 
and  through  the  mechanism.  It  is  more  difficult  to 
cool.  The  objections  mentioned  in  the  preceding  para- 


SELECTION   OF   TYPE  101 

graph  regarding  size  and  weight  of  parts  apply  equally 
well  here. 

The  multiple-cylinder  machine  is  lighter,  smaller,  carries 
a  much  smaller  fly-wheel,  and  as  there  is  a  more  continuous 
and  steadier  turning  effort,  the  engine  can  be  better  bal- 
anced. However,  the  multiple-cylinder  engine  must  be 
better  designed  and  constructed  so  that  each  cylinder  will 
do  its  share  of  the  work. 

Whenever  an  engine  is  to  be  designed  for  a  certain  class 
of  work  the  designer  must  decide  for  himself  how  far  the 
various  advantages  and  disadvantages  enumerated  in  the 
foregoing  apply  and  be  governed  accordingly. 

140.  SMALL  UNITS  vs.  LARGE  UNITS. — A  word  might 
here  be  said  about  installing  several  engines  in  place  of 
one  unit  where  a  certain  amount  of  power  is  wanted.  If, 
for  example,  1,200  H.-P.  is  required  in  a  power  station, 
there  are  many  advantages  (aside  from  the  question  of 
cost,  which  is  usually  less)  in  having,  say,  four  units  of 
300  H.-P.  each  in  place  of  one  unit  of  1,200  H.-P.  If  any- 
thing happens — and  things  are  bound  to  happen — and  one 
engine  must  be  stopped  for  a  while,  the  others  will  continue 
running  and  the  entire  plant  is  not  put  out  of  commission. 
Many  electric-light  and  power  stations,  pumping  stations, 
etc.,  have  installed  a  number  of  smaller  units  in  preference 
to  one  of  larger  size. 


CHAPTER  XVI 

DETERMINATION   OF  THE   PRINCIPAL   DIMENSIONS 

141.  POWER. — The  power  of  a  gas  engine  depends  prin- 
cipally upon  the  volume  of  air  (or  air  and  gas)  that  it  can 
handle  in  a  given  time — say  the  number  of  cubic  inches 
per  H.-P.  per  minute.    Fuel  requires  a  certain  amount  of 
air  for  complete  combustion  and  the  greater  the  amount  of 
air  handled  per  minute  the  greater  the  amount  of  fuel  that 
can  be  burned,  and  the  greater  the  amount  of  heat  that  is 
liberated  and  converted  into  mechanical  energy  in  a  given 
time.    After  having  decided  upon  the  type  and  power,  then 
the  cylinder  bore,  piston  stroke,  and  R.P.M.  can  be  deter- 
mined.   A  4^  x  4  \"  (bore  and  stroke)  four-cylinder  auto- 
mobile engine,  running  at  1,200  R.P.M.,  will  develop  as 
much  'power  as  an  8  x  12"  single-cylinder  engine  running  at 
about  400  R. P.M.,  if  the  fuel  is  the  same,  since  the  volume 
of  air  handled  per  minute  in  each  case  is  nearly  the  same. 
The  power  developed  by  an  engine  of  a  certain  bore  and 
stroke  depends  to  a  certain  extent  upon  the  fuel  used. 
An  engine  running  on  natural  gas,  a  rich  fuel,  will  develop 
more  power  than  when  running  on  blast-furnace  gas. 

142.  COMPRESSION. — The  allowable  compression  varies 
with  the  method  of  handling  the  fuel  and  the  method  of 
cooling.    Where  a  charge  of  air  and  fuel  is  compressed  in 
the  cylinder  the  danger  of  pre-ignition  puts  a  practical  limit 
on  the  degree  of  compression.    The  methods  of  increasing 
the  compression  limit  by  more  extensive  cooling,  adding  a 
surplus  of  air,  etc.,  have  already  been  discussed.    In  the 
smaller  engines  the  compression  ranges  from  75  to  100  Ibs. 

102 


DETERMINATION   OF   PRINCIPAL   DIMENSIONS       103 

per  sq.  in.  abs.,  while  in  the  larger  engines  operating  on 
lean  gases  the  pressure  runs  from  150  to  200  Ibs.  For  small 
engines  operating  on  liquid  fuels,  or  illuminating  gas,  75 
to  90  Ibs.  is  about  right. 

When  the  compression  pressure  has  been  decided  upon, 
the  size  of  the  compression  space  can  be  calculated  from 
the  curves  given  in  the  next  chapter,  after  the  bore  and 
stroke  are  known. 

143.  PISTON  SPEED. — The  piston  speed  is  limited  by  the 
mechanical  construction  and  the  strength  of  the  materials 
used.    Too  great  a  speed  will  increase  the  inertia  and  other 
forces  beyond  safe  limits,  and  will  also  cause  undue  wear. 
Some  average  speeds  are  given  below: 

Small  stationary  engines 600  ft.  min. 

Medium       "     *          "      800       " 

Large  "  "      900       " 

High-speed  engines 900 

Automobile  and  marine  engines  are  sometimes  run  at 
piston  speeds  of  1,200  ft.  per  min.,  and  even  more,  but 
such  excessive  speeds  cannot  be  recommended. 

The  piston  speed  means  the  total  piston  travel  back  and 
forth. 

144.  CYLINDER  VOLUME  ;  AIR  REQUIRED  PER  H.-P. — The 
air  required  in  cubic  inches  per  H.-P.  minute  varies  with 
the  fuel  and  assumed  thermal  efficiency.    Stationary  gas 
engines  are  usually  guaranteed  by  the  builders  to  develop 
their  rated  power  on  10,000  B.T.U.  per  H.-P.  hour,  which 
would  mean  an  efficiency  of  about  30  per  cent  for  the  I.H.P., 
and  25  per  cent  for  the  B.H.-P.    The  amount  of  air  required 
for  the  combustion  of  carbon,  assuming  complete  combus- 
and  a  thermal  efficiency  of  100  per  cent,  would  be  about  900 
cu.  in.  per  H.-P.  minute.    Assuming  an  actual  efficiency  of  20 
per  cent,  then  five  times  this  amount,  or  4,500  cu.  in.,  would 
be  required.     The  actual  volume  of  air,  or  air  and  gas, 
handled  per  H.-P.  minute  varies  in  different  engines  from 


104  GAS-ENGINE   THEORY   AND   DESIGN 

5,000  to  6,000  cu.  in.  Where  an  overload  capacity  is  de- 
sired the  cylinder  volume  must  be  increased.  The  amount  of 
air  required  can  readily  be  figured  from  the  calorific  power  of 
the  fuel  and  the  air  required  for  complete  combustion  of  1 
Ib.  of  the  fuel.  For  example,  1  H.-P.  requires  42.4  B.T.U. 
per  minute,  when  an  efficiency  of  25  per  cent  is  assumed 
170  B.T.U.  are  required  and  enough  air  must  be  furnished 
to  burn  this  fuel,  plus  a  certain  percentage  for  fluid  losses 
(leakage,  wire-drawing,  etc.)  and  for  an  overload  capacity. 

The  total  cylinder  volume  will  be  the  piston  displace- 
ment plus  the  compression  space. 

145.  BORE  AND  STROKE. — Having  found  the  amount  of 
air  to  be  handled  per  minute,  and  the  piston  speed  in  feet 
per  minute,  the  bore  and  stroke  can  readily  be  computed. 
For  example,  if  the  piston  speed  is  taken  as  900  feet,  and 
the  engine  is  desired  to  run  at  450  R.P.M.,  the  stroke  will 
be  12".  In  a  four-cycle  engine  the  cylinder  volume  would 
then  be  the  amount  of  air  required  per  minute  in  cu.  in. 

450  X  12 
divided   by 

The  ratio  of  bore  to  stroke  can  be  varied  to  suit  conditions. 
In  the  high-speed  engines  this  ratio  is  usually  1  to  1,  some- 
times 5  to  4,  although  of  late  there  is  a  tendency  to  make 
the  stroke  more  than  the  bore;  in  stationary  engines  of  ten 
1  to  1.2,  and  in  the  larger  engines  1  to  1.5.  This  last 
ratio  is  seldom  exceeded.  With  a  moderate  speed,  and  the 
stroke  somewhat  greater  than  the  bore,  there  will  be  more 
time  for  ignition,  combustion,  charging,  and  exhausting, 
and  a  better  fuel  efficiency  results.  In  the  case  of  an 
automobile  engine,  or  other  portable  motor,  power  with 
light  weight  is  the  most  important  consideration,  and  fuel 
economy  is  of  secondary  importance. 

The  cylinder  volume  is  frequently  figured  from  the 
M.E.P.,  etc.,  instead  of  from  the  amount  of  air  required 


DETERMINATION   OF   PRINCIPAL   DIMENSIONS       105 


for  an  engine  of  a  certain  power,  but  the  method  here  given 
is  the  simplest  for  beginners. 

146.  EFFECT  OF  ALTITUDE. — Since  the  density  of  the  air 
decreases  with  the  altitude,  a  gas  engine  will  develop  less 
power  at  a  high  altitude  than  near  the  sea  level,  and  the 
work  of  compression  will  be  somewhat  less.  The  effect  of 
altitude  can  readily  be  computed  from  Table  V. 

146a.— TABLE  V 

EFFICIENCIES    AT   DIFFERENT   ALTITUDES 


Altitude  in 
Feet 

Barometric 
Pressure  in 
Inches  of 
Mercury 

Barometric 
Pressure  in 
pounds  per 
Square  Inch 

Volumetric 
Efficiency  of 
Compression 
Per  cent 

Loss  of 
Capacity 
Per   cent 

Decreased 
Power 
Required 
per  Stroke 

0 

30.00 

14.75 

100 

0 

0 

1,000 

28.88 

14.20 

97 

3 

1.8 

2,000 

27.80 

13.67 

93 

7 

3.5 

3,000 

26.76 

13.16 

90 

10 

5.2 

4,000 

25.76 

12.67 

87 

13 

6.9 

5,000 

24.79 

12.20 

84 

16 

8.5 

6,000 

23.86 

11.73 

81 

19 

10.1 

7,000 

22.97 

11.30 

78 

22 

11.6 

8,000 

22.11 

10.87 

76 

24 

13.1 

9,000 

21.29 

10.46 

73 

27 

14.6 

10,000 

20.49 

10.07 

70 

30 

i6.1 

11,000 

19.72 

9.70 

68 

32 

17.6 

12,000 

18.98 

9.34 

62 

35 

19.1 

13,000 

18.27 

8.98 

63 

37 

20.6 

14,000 

17.59 

8.65 

60 

40 

22.1 

15,000 

16.93 

8.32 

58 

42 

23.5 

147.  HEAT  AND  POWER  UNITS,  ETC. — Below  are  given 
some  heat,  power,  and  other  units  which  are  very  con- 
venient in  heat-engine  calculations,  as  well  as  some  defini- 
tions of  the  terms  commonly  employed. 

Work  is  force  exerted  over  a  distance. 

Power  is  the  rate  of  doing  work;  the  time  factor  here 
comes  in. 

Energy  is  stored  work,  or  the  capacity  of  performing  work. 

Potential  energy  is  the  energy  stored  in  a  body  by  virtue 


106  GAS-ENGINE  THEORY   AND   DESIGN 

of  its  position.  Example:  A  stone  on  the  roof  of  a  building 
possesses  potential  energy  since  in  falling  it  can  do  work. 
A  deflected  spring  and  fuel  both  possess  potential  energy. 

Kinetic  energy  is  the  energy  possessed  by  a  body  by 
virtue  of  its  motion.  Example:  A  fly-wheel  possesses 
kinetic  energy,  as  does  also  a  bullet  when  in  motion. 

Conservation  of  energy. — Energy  cannot  be  destroyed  any 
more  than  matter.  Energy  cannot  be  produced  except  at 
the  expense  of  some  other  form  of  energy;  it  cannot  be 
created  or  destroyed,  but  it  can  change  its  form. 

1  horse-power  =  33, 000  ft.-lbs.  per  min. 

1          "  =746  watts. 

1          "  =42.4  B.T.U.  per  min. 

1          "          =2544.9  B.T.U.  per  hour. 

1          "          =0.746  kilowatt. 

1  B.T.U.         =778  ft.-lbs. 

1       "  =0.252  calorie. 

1  caloric  =3.968  B.T.U.  =  amount  of  heat  required  to 
raise  the  temperature  of  1  kilogram  of  water  1°  C. 

I  kilowatt  (Kw.)  =  1,000  watts. 
-1.34H.-P. 

Watts  =  volts  X  amperes. 

I  gallon  =  231  cu.  in. 

1  gallon  of  water  weighs  8.33  Ibs. 

1  cu.  ft.  of  water  weighs  62.425  Ibs. 

1  centimetre  =  2.54  ins. 

1  metre         =3.28  ft.  =39.37  ins. 

1  litre  =0.264  gal. 

1  kilogram    =2.204  Ibs. 

1  in.  =0.393  centimetre. 

1  ft.  =0.3048  metre. 

1  gal.  =3.785  litre. 

1  pound         =0.4536  kilogram. 

1  atmosphere  =  14.7  Ibs.  per  sq.  in.  =29.9"  of  mercury. 


CHAPTER  XVII 


FORCES   ACTING   IN   THE   GAS   ENGINE 

148.  In  order  to  calculate  the  weight  of  the  fly-wheel, 
diameter  of  crank-shaft,  etc.,  it  is  necessary  to  know  the 
character  and  magnitude  of  the  forces  acting  in  the  engine, 
and  while  these  cannot  always  be  determined  definitely, 
fairly  accurate  approximations  can,  as  a  rule,  be  made. 
The  acting  forces  are  due  to  the  gas  pressure  on  the  piston 
and  inertia  of  the  moving  parts. 

IDEAL  INDICATOR  DIAGRAM. — After  having  decided  upon 
the  power  and  cylinder  dimensions  an  ideal  indicator  dia- 


FIG.  46. 

gram  should  be  laid  out  and  kept  for  reference.    Much  can 
be  learned  from  a  comparison  of  the  actual  with  the  ideal 
diagram,  a  better  understanding  of  the  thermodynamic 
changes,  and  practical  limitations  of  practice,  will  result. 
In  Fig.  46  AB  represents  adiabatic  compression  accord- 
107 


108  GAS-ENGINE  THEORY  AND   DESIGN 

ing  to  the  law  PV1<41  =  P1V11'41,  BC  represents  the  rise  in 
pressure  at  constant  volume  during  explosion;  CD  repre- 
sents adiabatic  expansion  according  to  the  law  used  for 
compression;  DA  represents  the  drop  in  pressure  during 
exhaust. 

The  method  of  plotting  the  PVn  =  K  curve  is  given  in  the 
next  paragraph. 

The  method  of  calculating  the  theoretical  temperatures 
of  combustion  has  already  been  given,  viz. : 

Rise  in  temperature  =  -      — •• 
CvXW 

The  rise  in  pressure  for  BC  can  now  be  figured  since 
P  _T2 
Pt      T\" 

T,  the  absolute  temperature  at  the  point  B,  is  found  from 

PV      PV1 
the  formula  — =  =  —• 

149.  ACTUAL  INDICATOR  DIAGRAM. — In  order  to  figure 
the  strains  to  which  the  engine  parts  arc  subjected  the 
actual  indicator  diagram  is  necessary.  In  designing  a  new 
engine  such  a  diagram  must  be  forecast  as  accurately  as 
possible,  although  the  exact  diagram,  of  course,  can  only 
be  obtained  after  the  engine  has  been  built  and  operated. 
A  good  way  is  to  take  the  diagram  of  an  engine  of  the  same 
power,  compression,  and  operating  on  the  same  fuel,  when 
such  a  diagram  can  be  obtained.  Frequently  this  is  not 
possible,  and  in  such  a  case  the  following  information  will 
be  helpful : 

In  place  of  PV1-41  =  K  the  formula  PV1<35  =  K  is  used,  as 
this  approximates  more  closely  the  actual  compression  and 
expansion  curves. 


FORCES  ACTING   IN   THE  GAS   ENGINE 


109 


The  method  of  drawing  these  curves,  as  shown  in  Fig.  47, 
is  as  follows:  The  ordinates  represent  pressures.  The 
abscissas  represent  volumes.  If  a  line  OD  is  drawn  at  angle 
a,  then  1+tari  6  =  (l+tan  a)n.  Find  angle  6,  and  lay  off 
OC.  The  values  commonly  used  are  given  below: 


Factor  =  tan  b. 


n 

tan  a  =  .25 

tan  a  =  .15 

1.35 
1.41 

0.352 
0.37 

0.2075 
0.218 

The  smaller  the  angle  a  the  more  points  can  be  plotted. 
The  point  PV  being  known,  lay  off  XD  =  .25  XO;  lay  off 
CCX  =  .37  OC1  (for  n  =  1.41);  lay  off  XE  at  45°  and  draw 
EE*;  draw  PG  and  lay  off  GG1  at  45°;  draw  G1E\  giving 


FIG.  47. 


110  GAS-ENGINE  THEORY   AND   DESIGN 


FIQ.  48. 


10  20  30          40  50          60  70  80  90         100 


FORCES  ACTING   IN  THE  GAS   ENGINE  111 


FIG.  49. 


112  GAS-ENGINE  THEORY   AND   DESIGN 

one  point  on  the  curve.  Continue  in  the  same  way  to 
locate  the  other  points  and  draw  the  curve  through  these 
points. 

Fig.  48  is  a  curve  plotted  according  to  the  law  PV1'35  =  K. 

Fig.  49  shows  a  curve  plotted  according  to  the  law  PV1-41 
=  K,  also  a  volume-temperature  curve  plotted  according 
to  the  same  law. 

It  will  be  noticed  that  the  volume  first  decreases  rapid 
with  a  moderate  increase  in  pressure,  then  the  pressure 
increases  more  rapidly  until  the  curve  finally  becomes 
nearly  a  vertical  line. 

The  actual  explosion  pressures  will  vary  with  the  fuel, 
dilution,  compression,  etc.,  but  the  following  may  be  taken 
as  a  guide : 

Gasolene-explosion  pressure. . .  .  compression  pressure  x  4  to  4£ 
Illuminating  Gas  "         ....  "      x  3£ 

Producer  Gas  "        "      x  2J 

Blast-Furnace  Gas  "      x  2J 

The  compression  pressure  for  rich  fuels  is  about  90 
pounds. 

The  compression  pressure  for  lean  fuels  is  about  180 
pounds. 

150.  ANGULARITY  OF  CONNECTING-ROD. — If  we  assume 
that  the  fly- wheel,  and  consequently  the  crank-pin,  revolves 
at  a  constant  velocity  ratio,  i.e.,  at  a  constant  speed,  then 
the  velocity  of  the  slider  (piston  or  crosshead),  the  tan- 
gential effort  on  the  crank-pin,  and  the  side  thrust  on  the' 
slider  are  continually  varying,  due  to  the  angularity  of 
the  connecting-rod. 

In  the  following  figures  the  ratio  of  length  of  connecting- 
rod  to  stroke  has  been  taken  as  2  to  1,  the  crank-pin  is 
assumed  to  turn  at  a  constant  velocity  ratio,  and  the 
pressure  on  the  piston  to  be  constant. 

Fig.  50  shows  how  the  angularity  of  the  connnecting-rod 
affects  the  slider.  Angles  <p  and  o  are  equal. 


FORCES  ACTING   IN  THE   GAS   ENGINE 


113 


cdf  equals  the  velocity  of  the  slider  when  the  crank-pin 
is  at  6. 

cd  equals  the  velocity  of  the  slider  when  the  crank-pin 
is  at  b1. 

If  the  connecting-rod  moved  parallel  to  the  slider's  path 
the  vectors  would  be  equal. 


FIG.  50. 


While  the  crank-pin  moves  from  b  to  b2  (Fig.  51)  the 
velocity  of  the  slider  is  greater  than  the  velocity  of  the 
crank-pin  since  the  vector  extends  beyond  the  circle,  and 
the  velocity  of  the  crank-pin  is  taken  equal  to  cd.  The  posi- 


FIG.  51. 

tion  of  greatest  velocity  is  generally  assumed  to  be  when 
the  crank  and  connecting-rod  make  an  angle  of  90°.  This 
is  not  strictly  accurate,  but  close  enough  for  all  practical 
purposes.  This  velocity  condition  may  cause  a  knocking 
in  the  engine  if  there  is  any  play  on  account  of  the  inertia 
of  the  reciprocating  parts. 

8 


114 


GAS-ENGINE  THEORY  AND   DESIGN 


151.  SLIDER- VELOCITY  DIAGRAM. — In  Fig.  52  the  curve 
q/V°  shows  how  the  velocity  of  the  slider  changes  during 
the  stroke.  The  curve  is  laid  out  as  follows: 

Divide  the  semi-circle  bb10,  which  represents  the  travel  of 
the  crank-pin,  into  10  equal  parts;  cbl  is  the  crank,  and 


Fir..  .r>2. 


blal  the  connecting-rod;    when  the  crank-pin  is  at  61  the 
slider-pin  has  moved  from  a  to  a1 ;  draw  the  line  bld. 

Then  the  distance  cd  represents  the  velocity  of  the  slider 
for  the  position  shown,  as  compared  with  the  throw  (one- 
half  the  stroke)  which  is  taken  as  1. 


This  can  be  proved  by  means  of  the  instantaneous-centre 
and  similar  triangles.  The  proofs  for  this,  and  some  of 
the  following  statements,  will  not  be  given  as  they  are 
simple  geometrical  and  trigonometrical  propositions  and 
can  be  found  in  books  on  machine  design. 


FORCES  ACTING   IN  THE  GAS  ENGINE 


115 


Lay  off  a1/1  equal  to  cd,  and  continue  in  the  same  manner 
for  the  other  points  on  the  semi-circle. 

152.  TANGENTIAL-EFFORT  DIAGRAM. — A  part  of  the  force 
that  passes  through  the  connecting-rod  from  the  piston 
exerts  a  tangential  (turning)  effort,  and  a  part  of  the  force 
simply  exerts  a  side  thrust  against  the  crankshaft  bearing. 
In  Fig.  53  let  ac1  represent  to  some  scale  the  force  acting 
on  the  piston,  then  ab  represents  the  thrust  along  the  con- 
necting-rod. Now  let  be  represent  the  thrust  along  the 
connecting-rod;  draw  6/at  right  angles  to  cb,  i.e.,  tangent 


FIG.  54. 

to  the  circle ;  complete  the  parallelogram ;  then  bf  is  the 
tangential  pressure. 

In  Fig.  54  let  ai  represent  the  pressure  on  the  piston; 
draw  bld,  and  on  cb1  lay  off  blh  equal  to  ai;  draw  hg  parallel 
to  cd;  gh  equals  the  tangential  pressure  on  the  crank-pin 
when  at  the  point  61;  lay  off  a1/1  equal  to  hg,  and  continue 
in  the  same  manner  for  the  other  crank-pin  positions. 

The  curve  on  the  right  now  shows  how  the  tangential 
effort  varies  with  a  constant  pressure  on  the  piston. 

It  must  be  remembered  that  in  the  actual  engine  the 
piston  pressures  are  constantly  changing  and  the  actual 
pressures  from  the  indicator  diagram  must  be  used  in  lay- 
ing out  the  tangential-effort  diagram. 

152a.  OFFSET  CYLINDERS. — There  has  been  a  good  deal 
of  discussion  in  technical  journals  regarding  the  ad  van- 


116 


GAS-ENGINE  THEORY  AND   DESIGN 


tages  and  disadvantages  of  the  offset  cylinder.  Fig.  54a,  a 
small  offset,  is  used  by  several  automobile  builders.  The 
chief  advantages  claimed  are  a  more  equally  divided  side 
thrust  equalizing  wear,  and  a  more  direct  thrust  during  the 
working  stroke.  The  compression  stroke  is,  how- 
ever, performed  under  a  corresponding  disad- 
vantage. In  a  single-acting  steam  engine  this 
arrangement  is  advantageous,  but  in  a  gas  engine 
the  disadvantages  seem  to  balance  the  advan- 
tages fairly  well.  The  student  should  draw  the 
tangential -effort  diagrams  (taking  into  account 
the  inertia  of  the  reciprocating  parts)  for  an  offset 
of  \  and  J  of  the  stroke.  The  arrangement,  as  will  be  seen 
from  such  a  diagram,  is  really  a  quick  return  motion. 

153.  SIDE  THRUST  ON  THE  SLIDER. — In  Fig.  55  let  gbl 
represent  the  pressure  on  the  piston;  blal  represents  the 
thrust  along  the  connecting-rod;  then  gbl  represents  the 
side  thrust  on  the  slider. 


FIG. 54a. 


FIG.  55. 

Figs.  56  and  57  show  how  this  side  thrust  reverses  during 
the  compression  and  expansion  strokes.  The  inertia  of 
the  reciprocating  parts  enters  into  the  problem  of  sida 
thrust  and  the  thrust  may,  in  consequence,  reverse  during 
the  stroke. 

In  double-acting  engines  the  net  forward  pressures  must 
be  considered  in  laying  out  these  diagrams. 


FORCES  ACTING  IN  THE  GAS  ENGINE  117 

154.  RATIO  OF  CONNECTING-ROD  LENGTH  TO  STROKE. — 
It  can  readily  be  seen  that  the  longer  the  connecting-rod 
the  more  direct  the  thrust  on  the  crank-pin  until,  with  a 
connecting-rod  of  infinite  length,  the  thrust  would  be 
parallel  to  the  piston  axis.  The  advantages  of  a  more 
direct  thrust,  and  consequently  better  turning  effort,  are 
overbalanced  by  the  disadvantages  of  increased  inertia, 
friction,  longer  engine,  etc. 

The  ratio  of  connecting-rod  length  to  stroke  varies  from 
2  to  1  in  high-speed  engines  to  3  to  1  in  slow-speed  engines. 


1 

FIG.  56.  FIG.  57. 

A  ratio  of  slightly  more  than  2  to  1  is  commonly  used  in 
automobile  engines,  while  in  stationary  engines  2|  to  1  is 
the  usual  practice. 

155.  INERTIA. — A  force  tends  to  change  the  state  of  a 
body  with  respect  to  rest  or  motion. 

Inertia  is  the  property  of  a  body  by  virtue  of  which  it 
tends  to  continue  in  its  state  of  rest  or  motion  until  acted- 
upon  by  some  force. 

The  inertia  of  the  reciprocating  and  turning  parts  of  an 
engine  sets  up  forces  which  affect  the  turning  effort,  balance 
of  the  moving  parts,  and  sets  up  stresses  in  the  various 
parts.  The  turning  effort  should  be  as  constant  as  possible, 
and  the  balance  should  be  as  perfect  as  it  can  be  made, 
although  sometimes  balance  must  be  obtained  at  the  ex- 
pense of  turning  effort,  and  vice  versa.  Perfect  balance  is 
usually  impossible  in  the  ordinary  forms  of  construction 
since  reciprocating  parts,  for  instance,  can  be  balanced 
only  by  reciprocating  parts  and  not  by  turning  parts.  The 
unbalanced  forces  tend  to  rock  and  shake  the  engine.  In 


118  GAS-ENGINE  THEORY  AND   DESIGN 

a  vertical  engine  the  reciprocation  tends  to  lift  and  drop 
the  frame  alternately,  and  as  the  crankshaft  is  turning  in 
one  direction  the  frame  tends  to  turn  in  the  opposite  direc- 
tion. It  is  a  good  general  rule  that  the  frame  should  be 
made  stiff  and  the  reciprocating  parts  as  light  as  possible. 
The  inertia  is  much  greater,  of  course,  in  a  high-speed 
engine  than  in  a  slow-speed  engine,  the  difference  on  ac- 
count of  speed  may  be  10-1,  or  even  more. 

156.  INERTIA  OF  CONNECTING-ROD  AND  RECIPROCATING 
PARTS. — At  the  dead  centres  the  reciprocating  parts  and 
connecting-rod  may  be  assumed  to  be  at  rest.  During  the 
first  part  of  the  stroke  these  parts  oppose  the  piston  press- 
ure as  the  velocity  increases.  During  the  latter  part  of  the 
stroke,  as  the  velocity  decreases,  these  parts,  on  account 
d  of  their  velocity  and  inertia,  exert 
pressure  in  the  direction  of  the  piston 
travel.  The  connecting-rod  partakes 
of  both  reciprocating  and  turning  mo- 
tion, but  one-half  of  the  rod  is  usually 
c  assumed  (for  the  sake  of  convenience 

in  calculations,   and    because    nearly 
true)  to  have  reciprocating  motion  and  one-half  to  Jiavc 
turning  motion. 
The  following  formula  may  be  used: 


F  =.00034  WN2R(l 

F  =  . 00034  WN2R  (1-1/n) 

Where  F  is  the  inertia  at  the  beginning  of  the  stroke  in  Ibs., 
F1  is  the  inertia  at  the  completion  of  the  stroke  in  Ibs.,  W 
is  the  weight  of  the  reciprocating  parts  (piston,  etc.)  plus 
one-half  of  the  connecting-rod  weight  in  Ibs.,  N  is  the 
R.P.M.,  R  is  the  radius  of  the  crank-pin  circle  in  feet,  n 
is  the  ratio  of  connecting-rod  length  to  length  of  crank 
throw. 


FORCES  ACTING   IN  THE   GAS   ENGINE  119 

Fig.  58  is  an  inertia  diagram. 

F 
ac  =     - 

F1 

M-r 

0  is  the  point  at  which  the  inertia  effect  is  zero;  ab 
represents  the  length  of  the  stroke. 

A  is  the  piston  area,  so  that  ac  rep- 
resents the  inertia  force  in  Ibs.  per  sq. 
in.  of  piston  area. 
F      59  The   inertia   effect   is  zero   when  the 

angle  acb,  Fig.  59,  is  90°,  therefore  the 
point  O1,  Fig.  58,  can  be  computed  by  finding  the  dis- 
tance ac  in  Fig.  59  (ab2+bc2  =  ac2),  arid  subtracting  it 
from  the  sum  of  the  connecting-rod  and  crank  lengths. 
The  remainder  is  the  distance  ac  in  Fig.  58.  The  curve 
cod  can  now  be  drawn.  This  is  the  arc  of  a  circle 
passing  through  the  three  points  and  approximates  the 
true  inertia  curve  very  closely.  The  same  scale  of  pressures 
must  be  used  as  for  the  indicator  diagram. 

This  inertia  diagram  is  used  in  laying  out  the  tangential- 
effort  diagram  and  in  computing  the  weight  of  the  fly-wheel. 

157.  BALANCING. — In  a  multiple-cylin- 
der   engine    the    reciprocating    parts  — 
pistons  and  connecting-rods — should  be 
balanced  by  weighing  them  on   scales; 
the  crank-shaft  and  fly-wheel  should  be          FlG>  59a" 
balanced  on  knife-edges  as  shown  in  Fig.  59a.     The  im- 
portance of  this  balancing  is  obvious. 

To  show  how  light  the  reciprocating  parts  are  made  in 
high-speed  engines  the  following  weights  of  the  Franklin 
air-cooled  automobile  engine  (four-cylinder,  28  H.-P.)  are 
given:  Weight  of  piston,  rings  and  pin  complete,  4  Ibs. 


120 


GAS-ENGINE  THEORY   AND   DESIGN 


6J  oz.;   weight  of  connecting- rod  complete  with  bearings, 
cap  screws  and  liners,  4  Ibs.  3  oz. 

157a.  COUNTERBALANCING. — The  connecting-rod  and 
crankshaft  may  be  balanced  as  shown  in  Fig.  60.  Here 
r  =  r1  =  distance  to  centre  of  gravity.  The  weight  of  C 


I? 


FIG.  60. 


FIG.  61. 


FIG.  62 


FIG.  63. 


FIG.  64. 


equals  the  weight  of  the  crank.     Then  only  the  crank-pin 
and  half  of  the  connecting-rod  remain  to  be  balanced. 

W  =  W1+  W2 

Where  W  =  balance  weight  in  Ibs.,  W1  =  weight  of  crank- 
pin,  W2  =  weight  of  half  connecting-rod. 

The  proper  way  to  balance  is  as  here  shown — with  the 
balance  weights  in  the  same  plane  with  the  cranks.  The 
method  of  putting  the  weight  on  the  fly-wheel  rim  (in  an- 


FORCES   ACTING   IN   THE   GAS   ENGINE  121 

other  plane)  is  not  good.  In  order  to  make  the  latter 
method  effective  there  should  be  two  fly-wheels,  and  balance 
weights  should  be  placed  in  each  one.  This  throws  out  of 
balance  a  rotating  part  which  would  otherwise  be  balanced. 

In  engines  having  three 
or  more  cylinders,  especially 
those  running  at  high  speeds, 
balance  weights  are  often 
omitted.  The  crank-pins  here 
are  120°  and  90°  apart. 

In  Fig.  61  there  is  not  a  per- 
fect balance  since  the  inertia 

curves  (Fig.  58)  cross  and  are  not  the  same  for  the  cylinder  po- 
sitions shown.  There  is  also  a  couple  due  to  the  distance  d. 

In  Fig.  62  the  couple  is  zero,  but  the  forces  are  not  per- 
fectly balanced. 

I  ^^  f^~^~  *  In  Fig.  63  the  crank-pins  are  120° 
apart,  the  forces  are  balanced,  but 
there  is  a  couple. 

In  Fig.  64  there  are  no  couples,  but  there  are  free  forces. 

In  multiple-cylinder  engines  care   should  be  taken  to 
have  all  pistons,  connecting-rods,  etc.,  weigh  the  same. 

158.  FOUR-CYCLE  ENGINE  DIAGRAMS. — For  a  better  under- 
standing of  the  preceding  the  following  diagrams  are  given: 
Fig.  65  is  an  indicator  card  from  a  four-cycle 
single-cylinder  engine. 


FIG.  67. 


Fig.  66  is  the  inertia  diagram  of  the  reciprocating  parts ; 
Fig.  67  is  a  diagram  of  the  piston  pressures  for  the  com- 
plete cycle,  four  strokes. 


122 


GAS-ENGINE   THEORY  AND   DESIGN 


Fig.  68  is  the  combined  piston  pressure  and  inertia  dia- 
gram. 

Fig.  69  is  the  tangential-effort  diagram  derived  from  the 
preceding. 


FIG.  68. 


FIG.  69. 


In  the  case  of  a  two-cycle  engine  the  pump  diagram 
must  be  combined  with  the  pressure  diagram. 

In  the  case  of  a  multiple-cylinder  engine,  the  methods 
followed  are  the  same  as  in  the  foregoing. 


CHAPTER  XVIII 

DESIGN  AND   DIMENSIONS  OF  PARTS 

159.  REMARKS  ON  DESIGNING. — The  designing  of  a  gas 
engine,  or  of  any  other  machine,  is  largely  a  matter  of  ex- 
perience and  judgment.  The  final  form  and  proportioning 
of  parts  is  the  result  of  test.  A  number  of  formulas  will  be 
given  in  the  following  paragraphs,  but  machinery  cannot 
be  designed  by  formulas  alone.  The  author  has  frequently 
found  that  empirical,  and  other,  formulas  would  sometimes 
come  within  500  per  cent  of  the  correct  result.  Designing 
consists  of  calculating  the  stresses  acting  on  and  strength 
of  a  part,  and  giving  it  the  form  best  suited  for  the  use  to 
which  it  is  to  be  put.  The  designer  should  check  up  his 
calculated  dimensions  with  practice  as  far  as  possible. 
The  general  points  to  bear  in  mind  are: 

(a)  Each  part,  so  far  as  possible,  should  have  only  one 
duty  to  perform ; 

(6)  Each  part  should  be  as  simple  in  form  as  it  is  possible 
to  make  it; 

(c)  The  various  parts  should  be  easy  to  manufacture; 
this  includes  pattern-making,  foundry  and  machine-shop 
work; 

(d)  The  parts  should  be  easy  to  assemble  and  require 
little  fitting; 

(e)  The  wearing  surfaces  must  admit  of  easy  adjustment 
and  replacement; 

(f)  The  action  of  every  moving  part  should  be  positive; 

(g)  Needless  to   say,   the   material   and   workmanship 
should  be  good. 

123 


124 


GAS-ENGINE  THEORY   AND   DESIGN 


FIG.  70. 


The  large  horizontal  engines  developing  up  to  several 
thousand  horse-power,  with  their  heavy  moving  parts  and 
high  combustion  pressures,  present  difficulties  in  the  design 
and  building  which  are  peculiar  to  this  type  alone.  The 
light,  high-speed  engine  presents  difficulties 
of  altogether  another  nature.  Each  type  re- 
quires a  careful  study  of  the  difficulties  pecul- 
iar to  it. 

160.  MATERIALS  OF  CONSTRUCTION. — The 
physical  characteristics  of  the  metals  used  for 
the  various  parts  must  be  studied  in  order  to 
make  the  design  practical.  The  greater  part  of 
the  gas  engine  consists  of  castings,  and  a  knowledge  of  the 
general  behavior  of  iron  during  melting,  casting,  and  cooling 
is  essential.  The  pattern-maker  must  make 
•  the  proper  allowance  for  shrinkage,  but  it  is  the 
designer's  province  to  give  the  part  such  a 
form  that  strong  castings  with  minimum 
shrinkage  strains  will  result.  In  a  casting 
having  a  cross  section  similar  to  Fig.  70,  the  thin  part  cools 
quickly  and  the  metal  hardens  and  becomes  set.  The 

heavy  part  cools  slowly, 
the  surface  cools  first  and 
hardens,  then  the  interior 
cools  and  shrinks  and 
tends  to  draw  in  the  outer 
portions  which  have  al- 
ready cooled,  creating 
shrinkage  strains  and 
making  the  interior  por- 
ous. These  shrinkage  strains  weaken  the  metal  so  that 
the  casting  may  break  when  subjected  to  only  moderate 
strains.  Sharp  corners  where  the  thin  part  joins  the 
heavy  part  are  also  a  source  of  weakness.  Very  thin  and 


FIG.  71. 


FIG.  72. 


DESIGN   AND   DIMENSIONS   OF   PARTS 


125 


very  heavy  sections  should  be  avoided  if  possible.  The 
section  should  be  uniform  throughout,  or,  where  one  sec- 
tion must  be  thinner  than  another,  the  change  should  be 
gradual,  as  shown  in  Fig.  71.  The  fly-wheel,  Fig.  72,  fur- 
nishes a  good  example  of  the  strains 
set  up  in  cooling.  The  arms  cool 
first.  The  rim  cools  slowly  and 
tends  to  pull  out  the  arms,  putting 
them  under  tension.  The  slower 
the  castings  cool  the  stronger  they 
will  be.  If  the  design  is  good,  then 
the  matter  of  poor  castings  is  up  to  the  foundry.  Scrap  iron 
is  cheap,  and  some  of  it  in  the  frame  will  do  no  harm  if  cor- 
rectly mixed  and  poured,  but  poor  metal  and  poor  foun- 
dry work  will  result  in  a  return  of  engines  to  the  builder. 


FIG.  73. 


161.  ARRANGEMENTS  OF  CYLINDERS. — The  following  fig- 
ures show  various  arrangements  of  gas-engine  cylinders  in 
common  use  to-day,  and  require  but  little  explanation. 


u 

_n_ 

, 

FIG.  75. 


Fig.  73  is  a  single-acting  horizontal  engine,  two-  or  four- 
cycle. 

Fig.  62  is  a  horizontal  double-opposed  engine,  four-cycle. 


126 


GAS-ENGINE  THEORY  AND  DESIGN 


Fig.  74  is  a  single-acting  horizontal  tandem  engine,  four- 
cycle. 

Fig.  75  is  a  double-acting  horizontal  tandem  engine,  four- 
cycle. 

Fig.  76  is  a  horizontal 
twin -cylinder  engine, 
four-cycle,  cranks  at  90°. 
Fig.  77  is  a  horizontal 
double-acting  engine,  two- 
or  four-cycle. 

Fig.  78  is  a  horizontal 
double-piston  two-cycle 
engine,  Oechselhauser. 

Fig.   79    is  a  vertical 
single-acting  engine,  two-  or  four-cycle. 

Fig.  63  is  a  vertical  single-acting  engine,  three-cylinder, 
four-cycle,  cranks  at  120°. 


L 


n 


FIG.  77. 

Fig.  64  is  a  vertical  single-acting  engine,  four-cylinder, 
four-cycle,  cranks  at  180°. 


FIG.  78.  FIG.  79. 

Fig.  80  is  a  vertical  single-acting  engine,  six-cylinder, 
four-cycle,  cranks  at  120° ;  two  styles  of  crank-shafts  shown. 


DESIGN  AND   DIMENSIONS  OF  PARTS 


127 


Fig.  81  is  a  vertical  double-acting  engine,  two-  or  four- 
cycle. 
Fig".  82  is  a  four-cylinder  engine  with  cylinders  at  90°, 


FIG.  81. 


FIG.  82. 


making  a  very  compact  arrangement,  as  the  cylinders  can 
be  much  closer  together  than  in  the  ordinary  multiple- 
cylinder  type. 


128 


GAS-ENGINE  THEORY  AND   DESIGN 


161a. — DIRECTION  OF  ROTATION. — The  horizontal  engine 
always  "turns  over"  as  shown  in  Fig.  83.  The  vertical 
engine  may  rotate  in  either  direction,  usually  counter- 
clockwise when  the  observer  faces  the  fly-wheel.  In  the 


FIG.  83. 

case  of  a  twin-screw  boat  the  engines  are  right-  and  left- 
hand,  i.e.,  turn  in  opposite  directions,  the  propellers  usually 
"turning  over"  and  toward  each  other. 

162.  FRAMES. — The  drawings  here  given  are  simply  in- 
tended to  illustrate  principles. 


B 


FIG.  84. 


During  the  expansion  stroke  the  pressure  in  the  cylinder 
tends  to  force  the  cylinder  and  crank-shaft  apart  and  the 
metal  in  the  frame  which  resists  these  strains  should  be  as 
much  as  possible  in  a  straight  line.  The  conditions  should 
approach  those  shown  in  Fig.  83.  Figs.  84  and  85  are  plan 


DESIGN   AND   DIMENSIONS  OF   PARTS 


129 


views  of  the  large  horizontal  engine  shown  in  Fig.  83.  The 
outboard  bearing  in  Fig.  84  is  usually  of  the  type  shown 
in  Fig.  108.  The  European  builders  provide  three  bearings 
for  the  crank-shaft,  as  shown  in  Fig.  84,  while  several  Amer- 
ican builders  have  adopted  the  side  crank  shown  in  Fig.  85. 
In  the  latter  arrangement  there  is  a  considerable  side 
thrust,  and,  since  the  piston  pressures  are  heavy,  the 
strains  are  necessarily  great.  A  very  important  advantage 
is  that  there  are  only  two  bearings  to  keep  in  line — a  thing 
difficult  to  do  in  the  three-bearing  type.  When  a  fly-wheel 
weighs  a  trifle  of  fifty  or  eighty  tons,  this  matter  of  getting 


FIG.  85. 

bearings  true  is  an  exceedingly  difficult  one.  Unless  the 
bearings  are  amply  proportioned,  and  everything  is  in  line, 
and  the  shaft  is  stiff  so  that  there  is  but  little  deflection, 
the  bearings  will  quickly  wear  at  the  edges  and  hot  boxes 
will  result,  as  well  as  wobbling  of  the  fly-wheel.  A  fly-wheel 
may  burst  from  this  cause.  Another  point  which  here 
comes  in  is  that,  while  the  bearings  may  be  in  line  with 
the  machine  at  rest,  there  may  be  a  binding  of  the  crank- 
shaft when  the  engine  is  running. 

In  Fig.  85  PL  is  the  moment  acting  on  the  frame  at  the 
point  shown;  a  is  under  tension  and  a^  is  under  compression. 


130 


GAS-ENGINE  THEORY  AND  DESIGN 


The  moment  is  zero  at  the  centre  x,  and  greatest  at  the 
extremities. 

Fig.  86  shows  a  common  horizontal-frame  design  for  small 
engines.  The  letters  x  designate  the  usual  weak  points, 
which  are  somewhat  exaggerated  in  the  drawing.  These 
may  be  enumerated  as  follows: 

A  long  heavy  cylinder  supported  only  at  one  end; 

Too  little  metal  at  various  points  between  crank-shaft 
and  cylinder  end; 

Bearing  split  on  centre  does  not  take  thrust  correctly. 


FIG.  SO. 

Fig.  87  shows  the  usual  vertical  form  and  its  advantages 
can  be  seen  at  a  glance.  There  is  no  difficulty  about  taking 
care  of  the  pressures  tending  to  force  cylinder  and  crank- 
shaft apart  and  to  arrange  for  multiple-cylinder  construc- 
ion.  The  section  AB  is  treated  as  a  beam  fixed  at  both  ends 
and  loaded  at  the  centre.  The  frame  must  be  stiff  so  that 
there  will  be  no  bending  between  A  and  B  which  would 
tend  to  loosen  the  foundation  bolts. 


Maximum  fibre  stress  = 


Maximum  total  piston  pressure 
Area  smallest  section  of  frame. 


The  maximum  fibre  stress  per  square  inch  runs  in  dif- 
ferent engines  from  500  to  2,500  Ibs.  A  low  value  is,  of 
course,  to  be  preferred. 

General  rules  for  frame  design  are: 


DESIGN   AND   DIMENSIONS   OF   PARTS 


131 


The  greatest  stresses  should  be  resisted  by  metal  in  a 
straight  line; 

The  bending  moments  should  be  as  small  as  possible; 

The  frame  should  take  up  the  strains  so  that  there  is  no 
tendency  to  loosen  the  foundation  bolts; 

The  shrinkage  after  casting  should  be  as  even  as  possible 
all  the  way  through,  and  to  this  end  the  sections  should  be 
as  uniform  as  they  can  be  made.  Large  masses  and  small 
sections  are  to  be  avoided.  The  for- 
mer result  in  porous  castings  and  the 
latter  in  excessive  shrinkage  strains. 

The  materials  used  for  frame  con- 
struction are  cast  iron  and  cast  steel. 

163.  CYLINDERS. — The  cylinder  is 
subject  not  only  to  the  explosion 
stresses,  but  to  stresses  resulting  from 
rapid  temperature  changes  which 
cause  crystallization,  i.e.,  the  metal 
becomes  brittle  and  breaks.  Trouble 
may  be  also  caused  by  the  strains 
due  to  cooling  after  casting.  In 
designing  provision  must  be  made  to  allow  for  expansion 
due  to  heating  up  of  the  cylinder  while  running;  parts  that 
are  heated  to  a  considerable  extent  must  be  free  to  expand. 

The  cylinders  of  high-speed  engines  are  usually  bored  to 
nearly  the  finished  size,  then  set  aside  for  a  while  to  allow 
the  internal  strains  in  the  metal  to  adjust  themselves,  and 
then  finished.  The  cylinders  of  air-cooled  high-speed  en- 
gines are  usually  bored,  then  annealed  to  remove  all  in- 
ternal strains,  and  then  finished  to  size. 

The  cylinder  must  be  stiff,  but  not  too  heavy,  since  an 
excess  of  metal  not  only  increases  the  heat  loss,  but  tends 
to  overheat.  The  section  should  be  as  uniform  as  possible 
so  as  to  give  a  uniform  expansion. 


FIG.  87. 


132 


OAR-EXr.IXE   THEORY   AXD    DESIC.X 


The  allowance  for  re-boring  is  from  '"  up,  according  to 
the  size  of  the  cylinder.  In  light  engines  no  allowance  is 
made  for  re-boring.  The  cylinder,  cylinder  head  and  water 
jacket  should  be  subjected  to  a  hydraulic  test  for  leaks. 

Fig.  88  shows  cylinder,  water  jacket  and  head  cast  in 
one  piece. 

Fig.  89  shows  the  cylinder  and  water  jacket  in  one  piece, 
the  head  (also  water-jacketed)  being  cast  separate. 


J 


FIG.  88. 


FIG.  89. 


FIG.  90. 


Fig.  90  shows  cylinder,  water  jacket  and  frame  cast 
separate.  Such  a  cylinder  is  called  a  "liner,"  and  is  used 
in  the  larger  engines.  It  is  free  to  expand  and  can  be 
made  from  harder  metal  than  the  frame.  Better  castings 
can  be  secured  in  this  way. 

The  normal  strains  in  a  cylinder  are  tension  and  the 
greatest  pressures  result  from  explosion.  These  range  from 
300  to  800  Ibs.  per  sq.  in. 

In  figuring  the  strength  of  the  cylinder  to  resist  rupture 
as  shown  in  Fig.  91: 

P  =  p  X  a 
R  =  a1  X  s 
P  =  R 
where    P=the  maximum  piston  pressure. 

p=the  maximum  pressure  per  sq.  in. 


DESIGN   AND   DIMENSIONS   OF   PARTS 


133 


a=the  area  of  the  piston. 

R= resistance  offered  by  the  metal  to  P. 

a1  =  area  of  cylinder  wall  resisting  P  =  area  d  — 

area  d1. 

s  =  1,500  to  5,000  Ibs  per  sq.  in.     For  special  mate- 
rials the  values  are  higher, 
d  and  d1  =  outside  and  inside  diameters. 
The  greatest  strains  in  the  cylinder  are  those  tending 
to  produce  rupture  as  shown  in  Fig.  92,  since  the  highest 


FIG.  91. 


FIG.  92. 


explosion  pressures  are  reached  as  the  piston  starts  to  move 
out,  and,  therefore,  there  is  only  a  small  area  to  resist 
the  rupture  parallel  to  the  cylinder  axis. 

Here     P1  =  2(d  X  c)  X  p. 

a  =  c  X  t. 

K,l=  2a  X  s. 

F=  R1. 

Where    P1  =  maximum  pressure  tending  to  produce  rupture 

as  shown. 

d  =  inside  diameter  of  the  cylinder, 
t  =  thickness  of  cylinder  wall, 
c  =  distance  between  top  of  piston  and  cylinder 

head. 

R*  =  resistance  to  rupture. 

The  tension  is  greater  in  the  inner  layers  of  the  metal 
than  in  the  outer  layers,  a  fact  of  considerable  importance 
in  the  design  of  large  guns,  but  in  ordinary  gas-engine  prac- 
tice this  may  be  neglected. 


134 


GAS-ENGINE  THEORY   AND   DESIGN 


The  material  for  cylinders  is  a  close-grained  cast  iron 
which  can  readily  be  machined.  The  cylinders,  espe- 
cially where  liners  are  used,  may  be  harder  than  the  piston 
so  that  the  latter,  as  it  can  be  replaced  at  less  cost,  may 
take  the  greater  wear.  Iron  containing  about  1.5  per 
cent  of  silicon  possesses  considerable  tensile  strength 
with  a  fair  degree  of  hardness. 

164.  WATER  JACKET. — Water  jackets  are  made  thinner 
than  the  cylinder  walls  because  they  are  subjected  to 
little  pressure  and  no  wear.     The  water  jacket  is  propor- 
tioned with  a  view  to  obtaining  a  sound  casting.     In 

light  engines  it  may  be  f\  in.  thick 
if  cast.  Sometimes  the  jacket  is 
made  of  spun  copper.  In  hori- 
zontal engines  the  water  jacket 
has  to  carry  a  part  of  the  cylinder 
weight,  etc.,  and  is  webbed  and 
proportioned  accordingly.  The 
space  between  cylinder  and  jacket 
should  be  such  as  to  allow  of 
proper  cooling  with  a  moderately 
slow  flow  of  water.  Air  pockets 
must  be  avoided  and  provision 
made  for  draining  off  or  otherwise 
removing  impurities.  The  water  jacket  usually  comes  a 
little  below  the  lowest  point  of  piston  travel,  as  shown  in 
Fig.  88. 

165.  CYLINDER  HEAD. — The    cylinder  head    must  first 
of  all  be  rigid.     In  small  engines  the  thickness  is  about 
the  same  as  that  of  the  cylinder  wall.     Fig.  93  shows  a 
separate   water-cooled  head.     Sometimes  a  double    head 
is  ribbed,  but  the  ribs  arc  of  doubtful  advantage  since 
they    interfere    with    expansion.     Heads    are    sometimes 
corrugated  in  order  to  increase  the  stiffness.     The  shape 


IT 


FIG.  93. 


DESIGN   AND   DIMENSIONS  OF   PARTS  135 

of  the  head  depends,  of  course,  upon  the  valve  arrangement 
and  general  design. 

For  engines  of  large  diameter: 

t'  =  t  X  H 

t  =  thickness  of  cylinder  wall. 
t1  =  thickness  of  cylinder  head. 

Treated  as  a  uniformly  loaded  plate  fixed  at  the  edges, 
the  following  formula  (Grashof)  may  be  used: 


d     15 

2~\6 


Xp 


X  s 

d  =  cylinder  diameter, 
p  =  maximum  pressure  per  sq.  in. 
s  =  working  stress;  this  may  be  taken  the  same  as  the 
s  for  cylinder  thickness. 


FIG.  94. 

For  a  stiffened  (corrugated)  plate  t1  is  less  than  for  the 
non-stiffened  plate. 

In  Fig.  94,  a  shows  the  water  connection  between  cylin- 
der and  head;  b  and  c,  also  Fig.  93,  show  how  the  joints 
may  be  designed.  A  ground  joint  is  the  best  since  gas- 
kets require  frequent  screwing  down  of  cylinder  head  in 
order  to  keep  the  joint  gas-  and  water-tight. 

166.  BOLTS  AND  STUDS. — The  longer  a  bolt  the  better 
able  it  is  to  withstand  shocks.  If  too  long,  however,  it 
is  difficult  to  keep  the  nuts  tight.  A  hollow  bolt  is  also 
stronger  in  several  ways  than  a  solid  bolt.  Cylinder- 
head  bolts  should  be  fairly  close  together  so  as  to  prevent 


136 


GAS-ENGINE   THEORY   AND   DESIGN 


the  head  from  springing.  There  must  be  room  enough 
between  the  nuts  so  that  a  wrench  can  be  freely  used. 
Studs  may  be  used  where  there  is  little  or  no  occasion 
for  taking  them  out ;  otherwise  bolts  and  nuts  must  be  used, 
since  threads  in  cast  iron  wear  quickly. 
For  simple  tension: 

F  =  R 

R  =  n  X  a   xs 

where    F  =  maximum  total  piston  pressure. 
s  -  4,500  to  6,000  Ibs  per  sq.  in. 
n  =  number  of  bolts, 
a  =  area  of  bolt. 

Bolts  and  studs  are  always  made  larger  than  theoreti- 
cally necessary  so  that  they  will  stand  considerable  tight- 


FIG.  95. 


FIG.  96. 


FIG.  97. 


ening  up.     The  strains  due  to  screwing  down  a  nut  may 
be  calculated  by  the  principle  of  leverage  as  follows: 

St=  2-rp't 
where     S ,  =  stress  due  to  screwing  up. 

r  =  radius  of  force  applied  on  wrench-handle  in 

inches, 
p  =  pressure  of  force  applied  on  wrench-handle 

in  pounds. 

t  =  threads  per  inch  on  bolt. 
167.  VALVE  CAGES. — Valve  cages  should  be  so  arranged 


DESIGN   AND   DIMENSIONS   OF   PARTS 


137 


that  the  valve  seats  can  be  water-cooled  and  also  easily 
removed  for  re-grinding.  Several  ways  of  arranging  valves 
and  valve  cages  are  shown  in  the  following  three  figures. 
Fig.  95  shows  the  inlet  and  exhaust  passages  cast  in 
one  piece  with  the  cylinder.  This  arrangement  is  found 
in  many  light  engines.  The  caps  on  top  permit  the  ready 
removal  of  valves.  This  arrangement  makes  a  rather 
expensive  casting  and  increases  the  cooling  surface  in  the 
combustion  chamber.  The  inlet  valves  (in  the  case  of  a 
multiple-cylinder  engine)  are  placed  on  one  side  and  the 
exhaust  valves  on  the  other  side.  Two  camshafts  are 
therefore  required  for  operating  the  valves. 


FIG.  98. 

Fig.  96  shows  a  simple  and  compact  arrangement. 
A  more  direct  gas  flow  is  obtained.  The  cages  and  valves 
are  easily  removed.  There  is  no  extra  space  in  the  com- 
bustion chamber. 

In  Fig.  97  the  valve  cages  are  easily  removed.  In  the 
case  of  several  cylinders  one  camshaft  operates  all  valves. 

Pockets  in  which  burnt  gases  may  lodge,  or  places  where 
carbon  may  deposit,  must  be  avoided.  The  parts  sub- 
jected to  the  heat  of  the  exhaust  gases  are  often  made  of 
nickel  steel. 


138  GAS-ENGINE  THEORY  AND   DESIGN 

Fig.  98  shows  the  general  arrangement  of  cylinder, 
cylinder-heads,  valves,  and  water-cooled  piston  in  a  large 
horizontal  double-acting  engine. 

168.  PISTONS. — The  requirements  here  arc  that  the 
piston  must  not  spring,  it  must  distribute  the  side  thrust 
as  evenly  as  possible.  Above  6  in.  diameter,  pistons  are 
usually  web-stayed.  Large  pistons  are  both  webbed  and 
water-cooled.  The  top  of  the  piston,  which  is  hottest 
end  expands  most,  should  be  finished  off  smaller  than  the 
rest  of  the  piston  barrel  in  order  to  prevent  binding  when 
running  hot. 

Fig.  99  shows  a  simple  piston  design  for  small  engines. 


FIG.  99.  FIG.  100. 

Fig.  100  shows  a  piston  design  for  larger  engines.  The 
head  is  ribbed,  providing  additional  stiffness,  and  the 
bosses  for  the  piston  pin  are  also  stiffened  by  ribs,  help- 
ing to  obtain  an  even  distribution  of  side  thrust  along 
the  entire  piston  length. 

The  thickness  of  the  piston-head  may  be  calculated 
according  to  the  formula  for  the  thickness  of  the  cylin- 
der-head. 

The  material  used  for  pistons  is  a  close-grained  cast 
iron,  preferably  somewhat  softer  than  the  cylinder  metal 
so  that  the  piston,  which  is  easily  replaced,  may  take  the 
greatest  wear. 

The  ratio  of  piston  length  to  diameter  depends  alto- 
gether upon  the  design.  In  light  engines  it  may  be  1  to  1, 


DESIGN   AND   DIMENSIONS  OF   PARTS  139 

in  single-acting  engines  running  at  a  slow  speed  it  may 
be  2  to  1,  and  any  number  of  ratios  between  these  general 
limits  are  used.  The  longer  the  piston  the  less  the  side 
thrust  per  square  inch. 

169.  SIDE  THRUST  ON  PISTON  OR  CROSSHEAD. — In  Fig. 
101  let  L  represent  the  connecting-rod,  and  R  the  crank, 

then  p  =  P  tan  $ 

p  =  the  side  thrust  =R 

P  =  maximum  piston  pressure  ==  ab 

and  L  sin  #  =  R  sin  ft 

$  is  maximum  when  e  =  90° 

p  is  usually  taken  from  10  to  50  Ibs.  per  sq.  in.  of  pro- 
jected bearing  surface.     For  cross- 
heads  p  runs  up  to  100  Ibs.  and  more. 

170.  LOCATION  AND   DIAMETER 
OF  PISTON  PIN.  —  For  finding  the 

exact  location  of  the  piston  pin  the 
.,,,,,.  ,    '  FIG.  101. 

side-thrust  diagram  must  be  con- 
structed and  the  inertia  of  the  reciprocating  parts  also 
considered.  As  a  rule,  the  pin  is  so  placed  that  it  is  in 
the  centre  of  the  true  bearing  surface,  the  width  of  the 
rings  being  deducted  from  the  piston  length.  The  piston 
pin  is  sometimes  made  hollow  and  of  hardened  steel,  re- 
ducing the  weight  of  the  reciprocating  parts  and  providing 
a  path  for  the  lubricating  oil. 

The  piston  pin  may  be  treated  as  a  simple  beam  supported 
at  the  ends  and  uniformly  loaded.  For  a  hollow  pin  s 
equals  20,000  to  60,000  Ibs.  The  diameter  is  invariably 
greater  than  it  need  be  for  strength  in  order  to  obtain 
sufficient  bearing  surface.  Too  small  a  bearing  surface 
will  prevent  proper  lubrication  and  result  in  rapid  wear. 
The  bearing  surfaces  throughout  should  be  made  as  large 
as  possible,  i.e.,  as  large  as  is  consistent  with  good  design. 


140 


GAS-ENGINE   THEORY   AND   DESIGN 


1  X  d 

where    p  =  pressure  per  sq.  in.  projected  bearing  surface. 
P  =  maximum  piston  pressure, 
d  =  diameter  of  pin. 
1  =  length  of  pin  between  bosses. 

For  both  the  piston  pin  and  wrist  pin  (crosshead)  p 
should  be  kept  below  1000  Ibs.  if  possible,  although  in 
the  case  of  intermittent  pressures,  such  as  we  have  here, 
much  higher  pressures  (maximum)  are  allowable  than 
where  the  pressure  is  constant. 


FIG.  lOla. 


FIG.  lOlh. 


171.  PISTON-ROD  DIAMETER. — Where  a  crosshead  is 
used  (double-acting  engine)  the  diameter  of  the  piston 
rod  may  be  calculated  as  follows: 

d  =  b  D  x/~p~ 

where    d  =  diameter  of  the  rod. 
b  =  0.0140  for  steel. 
D  =  diameter  of  cylinder. 

p  =  maximum  unbalanced  pressure  per  sq.  in.  = 
difference  between  the  pressures  on  the  two  sides 
of  the  piston. 

17 la.  CROSSHEAD. — The  crosshead  design  follows  steam- 
engine  practice.  Fig.  lOla  shows  an  arrangement  which 
is  in  common  use.  The  bearing  surfaces  arc  lined  with 
babbitt  and  can  be  adjusted  for  wear.  The  guiding  sur- 


DESIGN  AND   DIMENSIONS  OF   PARTS  141 

faces  are  rounded  so  as  to  permit  a  certain  amount  of 
self-centring  of  the  mechanism. 

Since  in  double-acting  engines  the  pressure  on  the  cross- 
head  is  always  in  one  direction,  viz.,  downward,  the  cross- 
head  may  be  arranged  as  shown  in  Fig.  lOlb. 

172.  PISTON   RINGS. — Piston   rings   must   be   carefully 
made  since  their  function  is  to  prevent  leakage,  and  with 
poorly  fitting  rings  the  leakage  losses  Hai 

may  become  very  great.      The   rings  j     •  ] 

may  be  lap-cut  as  shown  in  Fig.  102.    Li 53 —      il  <* 

When  open  the  ring  is  larger  than 
the  inside  cylinder  circumference  by 
the  distance  2a.  In  order  to  have 
the  ring  a  true  circle  when  in  the 
cylinder  it  must  be  sprung  together 
and  then  machined  to  size.  It  is  bet- 
ter to  have  more  rings  with  the  dimen- 
sions b  and  d  small,  than  to  have  the 
,.  .. ,  .  ,  ,  ,,  FIG.  102. 

lewest  possible  rings  and  have  them 

wide  and  stiff.     For  eccentric   rings  the   following   pro- 
portions  represent  good  practice. 

d  =  .05  to  .04  D,  decreasing  as  D  increases. 

b-d 

c  =  fd 
where  D  is  the  inside  diameter  of  the  cylinder. 

Eccentric  rings  exert  a  more  uniform  pressure  all  the 
way  around  than  concentric  rings.     The  rings  are  put  in 
so  that  the  lap  joints  bear  against  the  cylinder  at  different 
points  and  pins  are  inserted  so  that  the  rings  cannot  turn. 
The  material  for  piston  rings  is  cast  iron. 

173.  CONNECTING-ROD. — The    ratio    of    connecting-rod 
length  to  stroke  has  already  been  discussed  in  Par.  154. 
The  connecting-rod  is  considered  as  a  strut  under  com- 
pression from  the  piston  pressure,  and  under  tension  from 


142 


GAS-ENGINE  THEORY  AND  DESIGN 


the  inertia  pressure.  The  greatest  piston  pressure  is  at 
the  beginning  of  the  stroke.  The  greatest  bending  moment 
due  to  inertia  acts  at  about  6/10  L  from  the  wrist-pin,  where 
L  is  the  distance  between  centres. 


FIG.  103. 

Fig.  103  shows  the  I-section  type  used  in  light  engines. 
The  cross  section  at  a  distance  2/3  L  may  be  calculated 
as  follows: 

_  f  X  P  X  L2 

Ex. 

I  =  moment  of  inertia  of  the  section  (see  Par.  183)  • 
f  =  factor  of  safety  for  compression  5-10. 
P  =  maximum  piston  pressure. 
E  =  modulus  of  elasticity  of  the  metal. 
L  =  length  of  connecting-rod  in  inches. 


where 


FIG.  104. 


The  ratio  of  a  to  b  may  be  1-2  or  2-3.     For  a  round 
connecting-rod,  Fig.  104,  the  following  formula  is  given: 

d-ajDLVp  +  C 


where      d  =  diameter  of  rod  at  §  L  from  piston  pin. 
D  =  piston  diameter  in  inches. 
L  =  length  connecting-rod  in  feet. 


DESIGN  AND  DIMENSIONS  OF  PARTS 


143 


p  =  maximum  piston  pressure  per  sq.  in. 
a  =  0.15    C  =  0.50"  for  a  fast  engine, 
a  =  0.08    C  =  0.75"  for  a  slow  engine. 


FIG.  105. 


FIG.  105a. 


For  a  rod  of  rectangular  cross  section,  Fig.   105,  the 
proportions  are: 


h-2t 

Connecting-rods  are  usually  steel  drop-forgings. 
174.  CRANK-SHAFT.  —  The    crank-shaft   is   subjected  to 
both    twisting    and    bending    moments.     The    maximum 


FIG.  106. 

twisting  moment  (Fig.  106)  is  obtained  from  the  tangential- 
effort  diagram.  The  bending  moments  are  due  to  both 
piston  pressure  and  fly-wheel  weight. 


144  GAS-ENGINE  THEORY   AND   DESIGN 

The  diameter  to  resist  twisting  may  be  calculated  as 
follows. 

For  a  solid  round  shaft: 


32 


d       .    !32xTr 


also  d  = 

For  a  hollow  round  shaft: 


32 

where    T  =  torsional  moment  in  inch-pounds. 

L  =  lever  arm. 

P  =  twisting  force. 

s  =  safe  shearing  stress  =  12,000  Ibs.  for  steel. 

J  =  polar  moment  of  inertia. 

r  =  radius  of  shaft. 

d  =  diameter  of  shaft. 

dt=  inside  diameter  of  hollow  shaft. 
The  diameter  to  resist  bending  is  figured  as  follows. 
For  a  solid  round  shaft: 


also  d=   .110-2  XT' 

\         s 

For  a  hollow  round  shaft: 

I  =  (d4— d.4)^ 
c          32  d 


DESIGN   AND   DIMENSIONS   OF   PARTS 


145 


where    T!  =  bending  moment. 
I  =  moment  of  inertia. 

—  =  moment  of  resistance, 
c 

The  resistance  of  a  shaft  to  bending  is  about  one-half  of 
that  to  resist  twisting. 

For  combined  twisting  and  bending: 
For  a  solid  round  shaft: 

T2  =  0.35  T,+  0.65  VT,'  +  T*  =  -^  ' 
For  a  hollow  round  shaft: 


4  r 

where    T2=  combined  twisting  and  bending  moments, 
r  +  TI=  out  and  inside  radius  of  hollow  shaft. 

A  hollow  shaft  is  lighter  for  the  same  strength. 

For  light  fly-wheels    d  =  about  £  D 

For  heavy  fly-wheels  d  =  about  \  D 
where  D  is  the  cylinder  bore. 

Since  the  crank-shaft  has  a  tendency  to  bend  when  run- 
ning it   must  be  designed    so    as  to 
possess  considerable  stiffness.  |pi 

In  single-acting  multiple-cylinder  en- 
gines the  crank-shaft  in  general  need 
not  be  larger  than  for  a  single-cylinder 
engine  since  the  maximum  stresses 
are  nearly  the  same.  Crank-shafts  are 
usually  steel  drop-forgings. 

175.  CRANK. — The  crank  is  treated 
as  a  cantilever  beam. 

The  area  of  the  cross  section  is  usual- 
ly from  1.1  to  1.4  the  area  of  the  crank-shaft  cross  section. 

176.  CRANK-PIN. — The   crank-pin    is   considered   as    a 
simple  beam  loaded  at  the  middle  and  subjected  to  bending. 

10 


FIG.  107. 


146 


GAS-ENGINE  THEORY  AND   DESIGN 


The  crank -pin  must  be  strong  and  rigid  and  the  bearing 
surface  must  be  large  enough  to  prevent  the  oil  from  being 
squeezed  out.  In  order  to  secure  the  required  bearing 
surface  the  diameter  is  invariably  made  greater  than  it 
need  be  for  strength;  1,400  Ibs.  per  sq.  in.  may  be  taken  as 
the  allowable  pressure.  The  crank-pin  is  frequently  made 
the  same  diameter  as  the  crank-shaft.  In  side-crank  en- 
gines the  pin  is  considered  as  a  cantilever  beam  (see  Par. 
183). 

177.  MAIN  BEARINGS. — The  length  of  the  main  bearings 
is  made  from  two  to  three  times  the  diameter  of  the  shaft. 


For  heavy  fly-wheels  an  outboard  bearing,  Fig.  108, 
should  be  used.  This  is  frequently  made  with  a  spherical 
joint,  as  shown,  to  allow  for  the  bending  of  the  shaft  in 
running.  In  order  to  prevent  rapid  wear  and  overheating 
the  main  bearings  should  present  ample  bearing  surface 
and  positive  lubrication  should  be  employed. 

When  the  main  bearing  is  made  in  two  pieces  the  general 
arrangement  shown  in  Fig.  108a  is  followed.  At  ss  thin 


DESIGN   AND   DIMENSIONS   OF   PARTS  147 

pieces  of  sheet  metal  or  other  material  (called  "shims") 
are  used,  so  that  the  cap  presses  against  the  lower  part 
when  it  is  screwed  down.  These  "shims"  are  also  used 
in  the  connecting-rod. 

In  large  engines  four-piece  bearings,  as  shown  in  Fig. 
108c,  are  frequently  used.  Since  the  main  bearings  must 
be  rigidly  supported  the  frame  should  be  arranged  (in 
the  case  of  a  horizontal  engine)  as  shown  in  108c  for  a 
large  engine,  or  1086  in  the  design  of  a  small  or  medium- 
sized  engine. 

178.  BEARINGS  AND  LUBRICATION. — The  reliability  and 
mechanical  efficiency  of  an  engine  depend  largely  upon 
proper  bearings  and  lubrication.  In  the  design  of  bearings 
the  following  must  be  considered : 

(a)  Friction. 

(6)  Lubrication  and  lubricants. 

(c)  Bearing-metals. 

(d)  Form  and  proportion  of  bearings. 

(e)  Mechanical  oilers. 
(/)  Stuffing-boxes. 

(a)  Friction. — In  the  case  of  solids,  friction  is  due  to 
the  unevenness  of  the  surfaces  in  contact.  The  metal  sur- 
faces appear  very  rough  under  the  microscope  no  matter 
how  much  they  have  been  polished.  These  projections 
oppose  the  sliding  of  the  surfaces  over  each  other.  Sliding 
friction  depends  upon  the  nature  of  the  surfaces  in  contact, 
the  speed,  the  amount  of  surface  in  contact,  the  nature  of 
the  lubricant,  etc.  The  smoother  the  surfaces  the  less  the 
friction,  the  greater  the  pressure  the  greater  the  friction,  etc. 

Fluid  friction  is  the  internal  friction  of  a  liquid  or  gas. 
In  the  case  of  a  lubricant  it  is  independent  of  the  pressure 
between  the  surfaces,  but  is  dependent  upon  the  area 
and  speed. 

(6)  Lubrication  and  Lubricants. — The   internal   friction 


148  GAS-ENGINE  THEORY   AND   DESIGN 

of  a  fluid  is  much  less  than  the  surface  friction  of  solids; 
therefore,  with  a  film  of  oil  between  the  rubbing  surfaces 
the  sharp  edges  cannot  engage,  and  the  work  lost  in  friction 
becomes  less.  When  there  is  a  film  of  lubricant  between 
the  metal  surfaces  they  obviously  cannot  touch. 

The  general  requirements  of  a  lubricant  are  as  follows: 
It  must  not  become  gummy;  it  must  resist  oxidation;  it 
must  not  corrode  metallic  surfaces;  it  must  be  able  to 
absorb  and  carry  away  the  heat  generated  by  friction;  it 
must  have  a  high  temperature  of  decomposition;  it  must 
have  sufficient  body  so  that  it  will  not  be  easily  squeezed 
out;  the  internal  friction  must  be  low. 

The  lubricant  should  be  suited  to  the  work  in  hand. 
For  light  pressures  and  high  speeds  a  thin  oil  is  best.  For 
great  pressures  and  slow  speeds  a  heavy  oil,  or  grease, 
should  be  used. 

Graphite  is  a  good  lubricant,  especially  if  fed  with  oil, 
but  in  the  case  of  the  gas-engine  cylinder  it  is  apt  to  short- 
circuit  the  spark-plug.  Where  there  is  no  danger  from 
this  source  it  forms  an  ideal  lubricant  since  it  is  not  affected 
by  high  temperatures. 

For  piston  lubrication  a  high-fire-test  oil  must  be  used. 
Unfortunately  even  the  best  of  these  oils  will  not  stand 
much  more  than  about  600°. 

Many  lubricating  oils  contain  acids  and  other  injurious 
substances.  A  good  method  of  testing  them  for  acid  is  to 
place  a  piece  of  polished  steel  in  the  oil  and  leave  it  there 
for  several  days.  Needless  to  say,  lubricating  oil  should 
be  thoroughly  filtered  before  being  used. 

(c)  Bearing-metals. — When  the  surfaces  are  highly 
finished  the  wear  will  be  greater  between  hard  metals,  as 
steel  on  steel,  than  when  one  metal  is  soft,  as  steel  on  babbitt. 
The  softer  metal  is  worked  into  shape  more  easily  and  by 
contact  with  the  hard  metal  is  smoothed,  decreasing  the 


DESIGN   AND   DIMENSIONS  OF   PARTS  149 

friction.  When  the  lubrication  fails  a  soft  metal,  like 
babbitt,  will  melt  and  run  out  without  injuring  the  shaft, 
whereas  with  brass  or  cast  iron  the  shaft  would  be  destroyed. 
The  bearing-metal  should  carry  its  load  without  distortion, 
and  must  not  heat  readily. 

Cast  iron  is  better  for  some  purposes  than  other  ma- 
terials. The  piston  furnishes  an  example  of  this.  Here 
cast  iron  wears  longer  than  brass  or  bronze. 

Brass  is  a  copper-zinc  alloy,  made  up  in  different  pro- 
portions and  sometimes  with  additional  ingredients. 

Bronze  is  a  copper-tin  alloy,  the  proportions  being  about 
90  per  cent  copper  to  10  per  cent  of  tin.  It  is  a  better 
bearing-metal  than  brass.  Bronze  is  used  where  the  press- 
ures are  too  high  for  babbitt. 

Phosphor  bronze  contains  a  small  amount  of  phosphorus 
which  improves  the  strength  and  ductility  of  the  alloy. 
This  bronze  is  used  extensively  for  bearings. 

Manganese  bronze  is  used  extensively  for  propellers, 
propeller  shafts,  and  salt-water  fittings  in  general,  since 
it  will  not  corrode  easily. 

Babbitt  is  a  copper-tin-antimony  alloy,  a  good  grade 
having  about  the  following  proportions:  tin  90  per  cent, 
antimony  7  per  cent,  copper  3  per  cent.  It  is  used  ex- 
tensively for  the  main  bearings  of  engines.  It  is  easily 
poured  in  place,  and  scraped  to  fit,  and  when  the  bearing 
overheats  it  will  melt  and  run  out  without  doing  any  injury. 
In  order  that  the  babbitt  may  hold  to  the  supporting  shell, 
grooves  must  be  provided  as  shown  in  Fig.  108.  The  bab- 
bitt is  sometimes  hammered  after  being  poured  in  order 
to  make  a  better  contact  with  the  shell. 

The  maximum  pressure  for  babbitt  at  slow  speeds  is 
given  as  1,000  Ibs.  per  sq.  in. 

The  maximum  pressure  for  bronze  is  5,000  Ibs.  per  sq.  in. 

The  maximum  pressure  between  steel  and  steel,  hard- 


150  GAS-ENGINE  THEORY  AND   DESIGN 

ened  and  polished,  as  in  ball-bearings,  etc.,  may  reach 
50,000  Ibs.  per  sq.  in. 

For  intermittent  pressures  higher  values  can  be  used 
than  for  constant  pressures. 

(d)  Form   and    Proportion   of  Bearings. — In    designing 
bearings  the  following  points  are  to  be  observed:    Find 
the   direction   and   magnitude   of  the 
forces  acting  on   the  bearing;   deter- 
mine the  safe  working  pressures  and 
speeds;  provide  means  for  forcing  the 
WMW^^          lubricant  between  the  bearing  surfaces ; 
provide  means   for    taking   up   wear. 
The  bearing-metal  must  not  change  its  shape  when  under 
load. 

In  Fig.  109,  if  the  lubricant  enters  at  A  it  will  take  the 
path  of  least  resistance  and  work  out  to  the  left  and  the 
rest  of  the  bearing  on  the  right  will  receive  no  lubrication 
at  all.  However,  if  the  oil  enters  at  the  centre,  B,  it  will 
meet  as  much  resistance  one  way  as  the  other  and  will 
work  in  both  directions.  If  grooves  are  now  cut  in  either 
the  shaft,  or  bearing-metal,  the  lubri- 
cant will  reach  all  parts  of  the 
bearing. 

When  the  motion  reverses,  the  oil 
has  a  better  chance  to  lubricate  than 
when  the  motion  is  always  in  one 
direction. 

If  in  Fig.  110  the  pressure  and  rota- 
tion are  as  indicated  by  the  arrows,  and  the  oil  enters  at 
A,  the  lubrication  will  be  poor  since  the  tendency  is  to 
squeeze  out  the  oil  where  the  pressure  is.  If  the  oil  is 
forced  in  at  B  under  pressure,  the  entire  bearing  will  be 
lubricated. 

In  Fig.  Ill  the  oil  enters  at  A  and  is  worked  outward 


DESIGN   AND   DIMENSIONS  OF   PARTS 


151 


by  centrifugal  force.    A  return  passage  is  provided  so 
that  the  oil  can  circulate. 

In  Fig.  112A,  if  the  bearing  is  made  thin  the  metal  will 
spring  and  bind  the  shaft  as  indicated  by  the  dotted  lines, 
causing  a  hot  bearing.  Fig.  112B  shows  a  heavier  bearing 


J^L 


FIG.  111. 


FIG.  112. 


which  offers  more  resistance  to  distortion,  and  shows  also 
how  the  metal  is  cut  away  at  aa  in  order  to  prevent  its 
seizing  the  shaft. 

(e)  Mechanical  Oilers. — Instead  of  allowing  the  oil  to 
feed  by  gravity  well-designed  machines  now  have  forced 
lubrication,  at  least  for  the  more  important  bearings. 
By  means  of  the  mechanical  oiler  a  dozen  or  more  bearings 
can  be  lubricated  with  oil  under  pressure  and  the  danger 
of  overheating  is  greatly  lessened.  A  saving  of  lubricating 
oil  also  results  since  there  need  be  no  waste. 

(/)  Stuffing-Boxes. — In  double-acting  gas  engines  a 
stuffing-box  is  required  for  the  piston-rod.  For  a  while 
this  was  quite  a  problem  since  the 
type  of  stuffing-box  used  on  steam 
engines  would  blow  out  or  the  pack- 
ing burn.  Fig.  113  shows  the  gen- 
eral construction  of  the  gas-engine 
stuffing-box.  A  number  of  cast- 
iron  rings  bear  lightly  around  the 
shaft  and  against  the  inside  of  the  casing.  Oil  is  pumped 
through  under  pressure.  The  entire  box  is  water-cooled. 

179.  VALVES  AND  VALVE  GEARING. — The  poppet  type 
of  valve  is  used  in  gas  engines  since  it  is  necessary  to  have 


FIG.  113. 


152  GAS-ENGINE   THEORY   AND   DESIGN 

a  tightly  fitting  valve  which  will  withstand  high  pressures 
and  temperatures.  The  following  must  be  studied  in  con- 
nection with  valve  design. 

(a)  Valve  proportions. 

(6)  Diameter  and  lift. 

(c)  Angles  of  valve  opening. 

(d)  Valve  gearing. 

(e)  Cams. 
(/)  Springs. 

(g)  Valve  passages. 

(a)  Valve    Proportions. — The    valve    proportions    vary 
much  in  different  designs.     The  thickness  t   (Fig.    114) 
must  be  sufficient  so  that  the  valve  will  not  spring.     This 
may  be  figured  according  to  the  formula  for  cylinder-head 

.  .      ,         thickness.     In  the  exhaust  valve 

t  is  frequently  made  larger  than 
in  the  inlet  valve.      As  a  rule  t 
runs  from  J  to  $  d,  decreasing  as 
the     diameter     increases.      The 
valve-seat  angle  is  usually  45°. 
The   distance   a   must    be  suffi- 
cient to  provide  a  good  bearing  surface  and  usually  runs 
from  -|  to  TV  d;  t1  may  vary  from  \  to  ±  d  for  a  solid  stem. 
The  metal  at  a1  permits  rcgrinding  of  the  valve. 

(b)  Diameter  and  Lift. — The  diameter  and  lift  are  fig- 
ured on  the  assumption  that  the  valve  is  fully  open  during 
the  entire  period  of  valve  lift  and  that  the  gases  are  moving 
in  and  out  at  a  constant  velocity.     In  a  high-speed  engine 
where  the  valve  is  lifted  from  800-900  times  every  minute, 
the  lift  is  made  as  small,  and  the  diameter  as  large,  as  pos- 
sible.   This  secures  a  smoother  action  and  reduces  the  ham- 
mering and  jumping  of  the  valve.     The  exhaust  valve  is 
sometimes  made  larger  than  the  inlet  valve,  but  usually 
they  are  of  the  same  size.     The  assumed  constant  speed 


DESIGN   AND   DIMENSIONS  OF   PARTS 


153 


FIG.  115. 


is  taken  as  100  ft.  per  second  for  the  inlet  valve,  and  85  ft. 
per  second  for  the  exhaust  valve  on  the  assumption  that 
the  exhaust  gases  are  going  out  at  atmospheric  pressure. 
Again,  a  constant  gas  speed  of  6,000  ft.  per  min.  is  as- 
sumed for  both  valves,  but  the  periods 
of  valve  opening  are  different.  The 
ratio  of  effective  lift  to  diameter  varies 
from  1-4  to  1-6  depending,  of  course, 
largely  upon  the  speed  of  the  engine. 
The  designer  must  be  careful  not 
to  confuse  the  total  valve  lift  h  with 
the  effective  valve  lift  h1.  These  two  are  quite  different 
as  can  be  seen  from  Fig.  115. 

To  illustrate  how  important  the  time  factor  is,  let  us 
take  a  four-cycle  high-speed  engine,  for  example: 
The  engine  runs  at  1,200  R.P.M. 
One  revolution  is  made  in  .05  sec. 
The  inlet  valve  is  open  during  180°  of  crank-pin  travel. 
Total  time  of  valve  opening,  .0250  sec. 
Valve  is  fully  open  about  .0050  sec. 
During  the  total  time  of  valve  opening  the  full  charge 
must  be  drawn  in. 

In  a  slow-speed  engine  the 
conditions  are  not  so  bad,  but 
in  a  two-cycle  engine  they  may 
be  even  worse. 

(c)  Angles  of  Valve  Opening. — - 
The  circle  in  Fig.  116  represents 
the  crank-pin  travel.  In  engines 
running  at  medium  and  high 
speeds  the  periods  of  valve  open- 
ing may  be  as  here  shown.  The  inlet  valve  commences  to 
open  10°  past  the  upper  dead  centre  and  closes  about  22° 
past  the  lower  dead  centre.  The  full  period  of  opening 


FIG.  116. 


154 


GAS-ENGINE   THEORY   AND   DESIGN 


is,  therefore,  212°,  or  106°  for  the  camshaft  which  revolves 
at  one-half  of  the  crank-shaft  speed.  The  reason  for  keep- 
ing the  valve  open  after  the  lower  dead  centre  has  been 
passed  is  on  account  of  the  inertia  of  the  gases  which  are 
coming  into  the  cylinder  at  a  high  velocity  and  continue 
to  come  in  even  after  the  piston  has  started  on  its  return 
stroke.  By  keeping  the  valve  open  a  larger  charge  passes 
into  the  cylinder. 

The  exhaust  valve  may  open  40°  ahead  of  the  lower 
dead  centre  since  the  tangential  pressure  at  this  point  is 


FIG.  117. 


FIG.  118. 


small  and  it  is  desirable  to  get  rid  of  the  hot  exhaust  gases 
as  rapidly  as  possible.  The  exhaust  valve  is  here  open 
during  226°,  or  113°  of  the  camshaft  travel.  These  angles 
are  varied,  of  course,  according  to  the  design. 

(d)  Valve  Gearing. — Fig.  117  shows  a  typical  valve  gear. 
Rotary  motion  is  transmitted  from  the  crank-shaft  C  to 
the  camshaft  K  by  gearing.  The  cam  bears  against  a 
roller  R  and  lifts  the  lever  L,  which  in  turn  lifts  the  valve. 
The  roller  reduces  the  friction  and  prevents  side  thrust 


DESIGN  AND   DIMENSIONS   OF   PARTS 


155 


against  the  valve  stem.  The  spring  S  holds  the  valve 
against  its  seat.  WW  are  water  spaces.  At  a  there  is  a 
little  clearance,  about  .01*,  so  that  the  valve  will  seat  be- 
tween strokes. 

The  motion  from  C  to  K  may  be  transmitted  by  three 
kinds  of  gearing — spur,  bevel,  or  spiral.  In  Fig.  118  A 
represents  two  spur  wheels  transmitting  motion  in  opposite 
directions;  B  represents  two  bevel  wheels  transmitting 
motion  at  right  angles;  C  represents  two  spiral  wheels 
transmitting  motion  at  right  angles,  but  in  different  planes. 
In  the  case  of  the  spiral  gear  a  reduction  of  2-1  can  be 
obtained  with  wheels  having  the  same  pitch  diameters. 


FIG.  119. 


FIG.  120. 


Unless  balanced  the  exhaust  valve  is  forced  open  against 
the  pressure  in  the  cylinder,  which  may  be  considerable. 
For  this  reason  large  exhaust  valves  are  usually  balanced  so 
that  the  valve  opens  against  a  low  resistance.  Figs.  119  and 
120  show  two  types  of  balanced  and  water-cooled  exhaust 
valves.  The  arrows  indicate  the  direction  of  water  flow. 

(e)  Cams. — General  rules  for  valve  openings,  which  are 
controlled  by  cams,  are  as  follows: 

The  valve  should  open  as  rapidly  as  possible. 

The  valve  should  close  as  rapidly  as  possible. 

The  period  of  full  valve  opening  should  be  as  long  as 
possible. 


156 


CIAS-ENGINE   THEORY   AND    DESIGN 


The  opening  and  closing  motion  should  be  as  smooth 
as  it  is  possible  to  make  it. 

Since  the  piston  velocity  changes  throughout  the  stroke 
the  velocity  of  valve  opening  and  closing  should  conform 
to  the  piston  velocity.  This  condition  can  only  be  approx- 
imated in  practice,  but  the  piston-velocity  diagram  should 
be  studied  in  connection  with  the  cam  curve.  The  average 
gas  velocity  has  been  given  as  6,000  ft.  per  min.  The 
maximum  velocity,  on  account  of  the  maximum  piston 
velocity  (since  the  piston  draws  in  and 
discharges  the  gases)  is  much  greater. 

Fig.  121  shows  the  usual  cam  outline. 
The  sides  of  the  cam  are  straight  lines 
tangent  to  the  base  circle  C.  Such  a 
cam  is  easily  machined  and  works  quite 
well  at  slow  speeds,  but  at  high  speeds 
the  valve  is  started  rapidly  from  rest 
at  d,  which  results  in  increased  wear,  the 
valve  is  also  apt  to  jump  at  e  and  el  instead  of  following 
the  cam  curve  closely,  and  this  causes  a  pounding.  Such  a 
cam  requires  a  much  stiffer  spring  than  the  curve  which 
will  be  described  next. 
The  angles  in  Fig.  121  are  usually  as  follows: 


FIG.  121. 


a 

b 

c 

Exhaust  valve  .            ... 
Inlet  valve  

42-45° 
35° 

22-24° 
20° 

42-45° 
35° 

A  cam  outline  which  will  give  a  smooth  motion  at  high 
speeds  should  start  the  valve  from  rest  gradually,  lift  with 
increasing  speed  and  then  decrease  the  speed  and  bring 
the  valve  to  rest  again  gradually.  A  body  acted  upon 
by  gravity,  falling  from  rest,  travels  with  a  uniformly 
increasing  acceleration  and  the  distances  passed  over  in 
successive  intervals  are  in  a  ratio  of  1,  3,  5,  7,  etc.  Fig.  122 


DESIGN   AND   DIMENSIONS   OF   PARTS 


157 


shows  the  method  of  laying  out  a  curve  following  this 
law.  The  distance  A  represents  the  arc  of  angle  a  on  the 
base  circle,  Fig.  121,  and  is  divided  into  ten  equal  parts. 
The  distance  h  equals  one-half  of  the  valve  lift.  The  line 
on  the  right  is  divided  into  parts  whose  lengths  are  1, 3, 5, 7, 
9,  respectively.  By  projecting  these  points  over  to  the  ver- 
tical lines  on  the  left  the  cam  curve  is  obtained.  The 


^ 

/ 
x 

x 
x 

x 

x7 

% 

X 

/ 

^^x 

X 

x 

X 

' 

_xl 

0     1 

a     i 

7     j 

9     10 

FIG.  122. 

upper  half  of  the  curve  is  simply  the  reverse  of  the  lower 
half.  The  straight  line  on  the  left  shows  how  the  cam 
lifts  the  valve  in  Fig.  121.  The  curve  may  now  be  used 
for  laying  out  the  cam  curve  proper  in  the  usual  manner. 
When  the  cam  bears  against  a  roller  care  must  be  taken  to 
draw  the  cam  curve  .so  that  the  centre  of  the  roller  will 
lift  according  to  the  curve  in  Fig.  122.  In  this  case  the 
actual  cam  curve  will  be  different  from  the  one  here  laid 
out. 

In  many  of  the  large  gas  engines  combinations  of  cams, 
eccentrics,  rods,  and  levers  are  used  for  operating  the  valves, 
and  these  arrangements  are  frequently  quite  complicated. 

The  ratios  of  base  and  roller  circles,  etc.,  are  as  follows: 

B  =  diameter  of  cam  base  circle. 

R  =  diameter  of  roller  circle. 

V  =  valve  lift. 


158  GAS-ENGINE   THEORY   AND   DESIGN 


Ratio  of  B  to  R  is  5-3 

Ratio  of  B  to  V  is  5-1  to  6-1  for  high  speed  and  4-1 
for  slow-speed  engines. 

The  diameter  of  the  base  circle  should  be  as  large  as  it 
can  conveniently  be  made.  Valves  are  drop  forgings  and 
are  frequently  made  from  nickel  steel  which  withstands 
high  temperatures  better  than  ordinary  steel. 

(/)  Springs.  —  The  function  of  the  spring  is  to  make  the 
valve  follow  the  cam  outline  closely  at  all  speeds  and  to 
keep  the  valve  closed  between  lifts.  The  spring  closes  the 
valve  against  inertia,  friction,  etc.  Helical  springs  made 
of  round  steel  wire  are  generally  used.  Many  turns  are 
used,  where  the  design  permits  this,  so  that  the  tension 
will  not  vary  to  any  extent  with  the  deflection. 

The  force  P  in  pounds  which  the  spring  must  exert  in 
order  to  close  the  valve  according  to  given  conditions, 
may  be  calculated  as  follows: 

2  w  h 

—  -  neglecting  friction 


32.2  X  t' 
where    w  =  weight  of  the  valve  and  stem. 

h  =  distance  it  moves  through  in  feet,  in  t  seconds. 

acceleration  =  ^  . 

\i 

The  diameter  of  the  wire,  diameter  of  coil,  etc.,  can  be 
now  found  in  tables  given  in  engineering  hand-books. 

The  pressure  per  sq.  in.  of  valve  area  may  run  from  .5 
to  5.0  Ibs.  and  even  more.  In  a  slow-speed  engine  the  re- 
quired pressure  may  be  1  Ib.  per  sq.  in. 

Where  throttling  or  cut-off  governing  is  used,  the  valves 
may  open  on  account  of  the  vacuum  in  the  cylinder  unless 
provision  is  made  for  locking  the  valves. 

(gr)  Valve  Passages. — The  diameter  of  inlet  and  exhaust 
pipe  for  each  cylinder  is  easily  figured  from  the  assumed 


DESIGN   AND   DIMENSIONS  OF   PARTS  159 

constant  gas  velocity.  Where  several  cylinders  are  sup- 
plied by  one  inlet  pipe,  as  in  Fig.  123,  the  area  of  the  main 
pipe  P  is  not  four  times  the  area  of  pipes  pp,  since  all  four 
cylinders  are  not  supplied  at  one  time,  but  in  a  four-cycle 
engine,  one  cylinder  after  another  receives  a  fresh  charge. 
Consequently  the  diameters  from  p  to  P  are  increased  in  a 
moderate  ratio.  There  should  be  as  few  turns  in  both  inlet 
and  exhaust  passages  as  possible  and  sharp  turns  must  be 


FIG.  123. 

avoided.  A  few  sharp  corners  will  have  the  same  effect  as 
decreasing  the  diameter  of  the  inlet  pipe  \  or  more. 

When  a  number  of  cylinders  are  supplied  from  one  car- 
bureter, trouble  is  experienced  in  supplying  each  cylinder 
with  an  equal  amount  of  the  mixture,  since  the  inlet 
passages  are  of  different  lengths,  and  hence  having  each 
cylinder  develop  the  same  amount  of  power.  For  this 
reason  several  carbureters  are  employed  by  some  builders, 
each  carbureter  supplying  two  cylinders. 

180.  FLY-WHEEL. — The  fly-wheel  regulates  the  running 
of  the  engine  while  the  governor  regulates  the  fuel  supply 
according  to  the  load.  The  fly-wheel  absorbs  energy  dur- 
ing the  expansion  stroke  and  furnishes  the  necessary  energy 
to  keep  the  engine  running  during  the  idle  strokes.  It 
regulates  the  speed  variations  per  revolution  due  to  the 
changing  pressures  on  the  piston  The  heavier  the  fly- 
wheel the  less  the  unsteadiness  in  running  will  be,  and  for 


160  GAS-ENGINE  THEORY   AND   DESIGN 

this  reason  the  wheel  is  made  much  heavier  than  it  need 
be  for  overcoming  the  idle  strokes.  While  the  fly-wheel 
is  giving  out  energy  it  slows  down  and  while  it  is  absorb- 
ing energy  it  speeds  up. 

W  v3 
The  kinetic  energy  of  a  moving  body  =  E  =  — - — 

o 

where  w  =  weight  of  body  in  pounds, 

v  =  velocity  in  feet  per  second, 
g  =  gravity  =  32.2. 

Let  El  represent  the  change  of  kinetic  energy  betwreen 
v{  and  v2 

where  vt  =  maximum  velocity  of  fly-wheel  rim 

v2=  minimum  velocity  of  fly-wheel  rim 

„     wv?    wv: 

then  E!  = 

and 

Let  us  assume  that  Fig.    124  is  the  tangential-effort 
diagram   for  the   expansion  stroke.     The   line   be  is  the 


b     A 


FIG.  124. 

M.E.P.  line.  While  the  piston  moves  from  6  to  61  the 
fly-wheel  is  giving  up  energy  equivalent  to  the  shaded 
portion  A,  and  consequently  the  wheel  slows  down. 
From  61  to  c1  the  fly-wheel  is  absorbing  energy  and  speeds 
up,  the  energy  absorbed  being  equivalent  to  the  shaded 
area  B.  From  cl  to  c  the  fly-wheel  is  giving  out  energy 
and  slows  down. 

E2  =  greatest  amount  of   energy  in  ft.-lbs.  above   the 
mean  (6-c)  =  B 


DESIGN   AND   DIMENSIONS   OF   PARTS  161 


then  W  = 


where  w  =  weight  of  rim. 

V  =  mean  velocity  of  rim  in  ft.  per  sec. 

v,  —  v2      greatest  change  in  velocity 
K  =  -^r  —  =  —  -si  —  r-  -  -  =  coefficient  of 

V  mean  velocity 

unsteadiness. 

In  the  case  of  several  cylinders  the  M.E.P.  is  greater 
and  the  area  B  is  consequently  smaller. 
K  =.03  to  .05  for  ordinary  work. 
.08  to  .10  for  light  fly-wheels. 
.01  for  textile  and  spinning  machinery. 
.002  to  .005  for  alternating  and  d.c.  drives. 
Empirical  formula  for  fly-wheel  weight  is 
110,000,000,000  XH.P. 

W    —  -  --  • 

K  x  d'  x  N3 

d  =  mean  diameter  rim  in  inches. 

N  =  R.P.M. 

The  allowable  rim  speed  for  cast-iron  wheels  is  about 
5,000  ft.-min.  Wheels  built  up  from  forged  and  rolled 
materials  can  be  run  at  much  higher  speeds  and  possess, 
among  other  advantages,  the  important  one  of  safety. 
Cast-iron  wheels  may  burst  from  overspeeding,  defective 
spots  in  the  casting,  and  wobbling  during  running  on 
account  of  bearings  being  out  of  line  or  worn.  Solid  cast 
wheels  are  stronger  than  wheels  cast  in  sections. 

The  safe  speed  for  a  cast  wheel  may  also  be  figured  as 
follows: 


-i.6JX 

\  W 


where     V  =  velocity  in  ft.  sec. 

w  =  weight  of  1  cu.  in.  of  rim  material  =  .260  Ibs. 
for  cast  iron. 
11 


162  GAS-ENGINE   THEORY   AND   DESIGN 

The  centrifugal  force  tending  to  produce  rupture  in  the 
rim  as  shown  in  Fig.  125  is 

_Wv' 

Cf~  gR 

where  R  is  the  mean  radius  in  feet. 

Cf 

T  =  tension  in  any  section  of  the  rim  =  -^- 

The  weight  of  the  arms  and  hub  usually  equals  one- 
third  of  the  total  weight  of  the  wheel  and  the  energy  stored 


FIG.  125. 

in  them  for  a  given  change  in  velocity  is  about  10  per  cent 
of  that  stored  in  the  rim. 

The  dimensions  for  the  cross  section  of  the  arms  near 
the  hub,  Fig.  126,  may  be  calculated  as  follows: 

d  =  0.l 


where    N  =  number  of  arms. 

D  =  diameter  of  cylinder  in  inches. 
B  =  width  of  rim, 

or  the  bending  strength  of  the  arms  is  made  equal  to  the 
twisting  strength  of  the  shaft. 

S  *  r3  ^  n  S.  I 
2  c 

S  =  shearing  strength  of  shaft. 
r  =  radius  of  shaft  in  inches. 


DESIGN  AND  DIMENSIONS  OF  PARTS  163 

N  =  number  of  arms. 

St=  tensile  strength  of  arms. 

—  =  section  modules. 

I  =  moment  of  inertia. 

C  =  distance  from  neutral  axis  to  the  outermost  fibre. 
The  arms  are  tapered  somewhat  toward  the  rim.     This 
taper  depends  altogether  upon  the  design.     In  the  case  of 


a  multiple-cylinder  engine  the  fly-wheel  is  smaller  than 
for  one  cylinder  since  the  area  A,  Fig.  69,  is  much  less. 

In  order  to  relieve  the  shrinkage  stresses  the  pattern 
may  be  made  with  a  split  hub  although  the  fly-wheel  rim 
is  solid. 

Large  wheels  are  made  in  halves  on  account  of  the  easier 
handling.  Fig.  126A  shows  such  a  wheel.  The  frame  is 


164  GAS-ENGINE   THEORY   AND   DESIGN 

cored  out  for  the  forged  joints.  The  hub  is  cored  out 
as  shown  in  126B,  so  as  to  relieve  the  shaft.  The  bolts, 
etc.,  are  also  relieved  as  shown. 

When  one  tapered  key  is  used  there  is  a  tendency  to 
throw  the  wheel  out  of  true,  as  indicated  in  Fig.  126C. 
In  order  to  avoid  this  three  keys  should  be  used  in  large 
wheels  as  shown  in  Fig.  126A.  It  is  of  the  utmost  impor- 
tance that  the  wheel  runs  true. 

A  fly-wheel  with  an  outboard  bearing,  Fig.  126E,  is  to 
be  preferred  to  an  overhung  wheel,  Fig.  126D.  A  heavy 
moving  part,  such  as  a  fly-wheel,  should  be  supported  be- 
tween two  bearings  whenever  practicable.  When  two 
overhung  wheels  are  used,  Fig.  126F,  the  crank-shaft  is 
subjected  to  a  greater  torsional  strain  than  when  one  wheel 
is  used  and  made  equal  in  weight  to  the  two  wheels. 

When  a  fly-wheel  is  made  in  halves  the  safe  tensile 
strength  of  the  bolts  should  equal  the  centrifugal  force 
in  the  rim. 

The  small  vertical  engine,  running  at  a  fairly  high  speed, 
is  often  discriminated  against  for  portable  and  other  pur- 
poses in  favor  of  the  much  heavier  horizontal  engine,  on 
the  plea  that  the  fly-wheels  on  the  vertical  engine  do  not 
possess  sufficient  "heft"  for  the  work  to  be  done.  In  the 
comparison  below,  the  data  was  taken  from  catalogues, 
the  only  change  being  in  the  speed  of  the  vertical  engine, 
which  has  been  increased  somewhat. 

Horizontal  engine,  6  H.-P.,  2  fly-wheels  of  200  Ibs.  each, 
mean  diameter  of  wheels,  3  feet,  speed,  300  R.P.M.,  total 
weight  of  engine  1 ,400  Ibs. 

Vertical  engine,  6  H.-P.,  2  fly-wheels  with  a  mean  diam- 
eter of  18",  speed  1,200  R.P.M.  In  order  to  have  the  same 
"heft"  as  in  the  first  case  the  wheels  should  weigh  60  Ibs. 
each.  The  total  weight  of  the  engine  would  then  be  a 
little  over  200  Ibs. 


DESIGN   AND   DIMENSIONS   OF   PARTS 


165 


Where  manufacturers  put  out  1-,  2-,  3-,  and  4-cylinder 
engines,  the  same  fly-wheel  is  generally  used  for  the  multi- 
ple-cylinder engine  as  for  the  single-cylinder  engine. 

181.  TACHOMETER. — The  tachometer  is  an  instrument 
for  recording  the  variation  in  speed  per  revolution.  Fig.  127 
shows  a  tachometer  record  covering  a  period  of  4  cycles,  or 
8  revolutions.  The  horizontal  distances  represent  cycles, 
while  the  vertical  distances  represent  variations  in  fly- 


FIG.  127. 

wheel  velocity  during  the  cycle.  An  electrical  tachometer 
will  greatly  exaggerate  these  velocity  variations.  The 
record  in  question  shows  that  the  engine  was  equipped 
with  too  light  a  fly-wheel  for  the  load.  The  fly-wheel 
weight  should  be  computed  not  only  for  full  load  but  for 
other  loads  as  well. 

182.  FOUNDATIONS. — The  method  of  building  founda- 
tions for  stationary  engines,  and  the  manner  of  securing 
the  foundation  bolts,  are  the  same  as  employed  in  steam- 
engine  practice  and  need  not  be  described  here.  When 
an  engine  is  installed  on  a  floor  it  should  be  placed  near 
a  wall,  preferably  in  a  corner,  in  order  to  lessen  the  bending 
of  the  floor  beams  and  decrease  the  vibrations.  When 
placed  on  a  built-up  foundation  the  engine  should  be  some 
distance  from  the  walls  of  the  building.  A  layer  of  min- 


166 


GAS-ENGINE   THEORY   AND   DESIGN 


eral  wool  or  tan  bark  underneath  the  foundation  will 
lessen  the  vibrations.  The  weight  of  the  foundation 
for  an  engine  with  heavy  fly-wheels  should  be  at  least 
four  times  the  weight  of  the  engine.  The  area  covered 
by  the  foundation  will  also  affect  the  matter  of  vibration. 
A  very  deep  foundation  covering  little  area,  or  a  founda- 
ation  which  covers  much  area  but  has  little  depth,  will  not 
give  the  best  results.  This  matter  of  vibration,  and  con- 
sequent shaking  of  the  building,  sometimes  becomes  a 
serious  one,  but  with  a  balanced  engine  and  a  fairly 
large  foundation  there  will  be  little  trouble. 

183.  STRENGTH  OF  MATERIALS. — The  following  matter 
is  given  for  the  convenience  of  the  designer.  Where 
special  materials  are  used,  such  as  nickel  steel,  chrome 
nickel  steel,  etc.,  the  strength  in  tension,  compression, 
etc.,  will  differ  more  or  less  from  the  values  given,  and  this 
fact  must  be  borne  in  mind  and  the  proper  values  sub- 
stituted. 

STRENGTH   OF   MATERIALS 


ULTIMATE  STRENGTH  S  IN  LBS.  PER  SQ.  IN. 


Cast  Iron 

20,000 
90,000 
20,000 
36,000 


Wrought  Iron. 

55,000 
55,000 
50,000 
50,000 


Steel. 

100,000 

150,000 

70,000 

120,000 


Tension 
Compression 
Shear 
Flexure 


450 
0.26 


WEIGHT  IN  LBS.  PER  Cu.  FT.  AND  Cu.  IN. 


480 

0.28 


490 
0.29 


FACTOR  OF  SAFETY  F. 


Steady  str 
Varying  ' 
Shocks 


15,000,000 


COEFFICIENT  OF  ELASTICITY  E. 
25,000,000  30,000,000 


DESIGN   AND   DIMENSIONS   OF   PARTS 

STRENGTH    OF    MATERIALS     (Continued) 

Q  FOR  COLUMNS 


167 


Cast  Iron  Wrought  Iron        Steel 

1  1  1 

5,000  36,000  25,000 

4  4  4_ 

5,000  36,000  25,000 


Both  ends 
flat  or  fixed 


Both  ends 
round 


~64 


32 


bd3 
12 


bd' 


V(d4-d,4^  Tr(d(-di4)  d2-d> 


16 


64 


32 


"16" 


12 


bd3-bldl3         bd3-b1d1 
Of/  12(6d  —  birf 


168 


GAS-ENGINE  THEORY   AND   DESIGN- 


STRENGTH  OF    MATERIALS     (Continued) 
BENDING  MOMENT  M 


4 

iM  =  Vfl 

1 

beam 

1            beam 

A 

loaded             A 
one  end 

M=^~            J 

~~  '  '  —  1  Cantilever          '//'\              ^ 

A        uniformly 
loaded 

^              WZ 
^%^      Fixed 

1       h«wn               /A 

^      beam 

uniformly         MSA 
loaded 

TV  7 

i        M  =  V 

Simple              ^l 

^P  loaded  at 
centre 

Fixed 

\           , 

_^_ 

A 

(- 

A      loaded  at          '/JVA 
centre 

p      loaded 

7T  (rf  4  —  rf,  4) 

TORSION-POLAR  MOMENT  OF  INERTIA 

-rfH     J    32          (\T~dr~7)     ^ 

32 

(see  Par.  174) 


Tension  Compression    Shear 


Flexure 


Torsion 


DESIGN   AND   DIMENSIONS    OF    PARTS  16! 

STRENGTH    OF    MATERIALS     (Continued) 

I  =  Moment  of  inertia  P  =  Total  stress  in  Ibs. 

R  =  Moment  of  resistance  I  =  Length  in  in. 

Ga  =  Square  of  least  radius  of  gyration        W  =  Weight  in  Ibs. 
A  =  Area  of  cross-section  in  sq.  in. 


For  tension,  compression,  or  shear 

where  I  does  not  exceed  10  diameters 


P  = 


AS 


Breaking  strength  of  beams     M  =  SR 

SA 
For  columns     P  = r?- 


FOR  A  SIMPLE  BEAM  OF  UNIFORM  STRENGTH 


At  0.1  I  Depth  =  0.45  d 

At  0.2  I  Depth  =  0.63  d 

At  0.3  I  Depth  =  0.77  d 

At  0.4  I  Depth  =  0.89  d 


CHAPTER  XIX 


GAS-ENGINE    MANIPULATION 

184.  Printed  instructions  arc  usually  furnished  by  man- 
ufacturers in  regard  to  starting,  stopping,  cleaning,  and 
taking  apart  their  engines,  as  well  as  directions  for  installing, 
etc.     The  principal  points  to  be  observed  will  be  briefly 
given. 

185.  STARTING. — In   starting  the   following   operations 
are  performed: 

Turn  on  the  fuel;  turn  on  the  current — where  electric 
ignition  is  used;  turn  on  the  cooling  water;  turn  on  the 


L: 


FIG.  128. 


FIG.  129. 


lubrication;  see  that  there  is  no  load  on  the  engine;  start 
the  engine ;  throw  on  the  load. 

186.  STARTING  DEVICES. — Small  single-cylinder  engines 
are  usually  equipped  with  starting-handles  as  shown  in 
Figs.  128  and  129.  The  engine  is  turned  over  in  the  direc- 
tion in  which  it  is  to  run. 

Considerable  force  is  required  to  overcome  the  resis- 
tance of  compression,  and  on  this  account  relief-cocks 
170 


GAS-ENGINE  MANIPULATION  171 

are  placed  on  the  combustion  chamber.  The  relief-cock 
is  opened,  the  engine  is  turned  over  in  the  direction  in 
which  it  is  to  run  and  after  two  or  three  revolutions  an 
explosion  should  take  place.  When  the  engine  is  running 
the  relief-cock  is  closed. 

High-speed  and  the  larger  stationary  engines  are  usually 
equipped  with  a  set  of  auxiliary  cams  on  the  exhaust- 
valve  camshaft.  The  camshaft  can  be  pushed  along 
so  that  the  auxiliary  cams  operate  the  exhaust  valves 
and  hold  them  open  during  a  part  of  the  compression 
stroke,  so  making  the  starting  much  easier. 

If  a  multiple-cylinder  engine  has  not  been  idle  long, 
especially  if  the  compression  is  good,  there  is  an  explosive 
charge  in  one  of  the  cylinders  and  the  wiring  can  be  so 
arranged  that  by  pushing  a  button  all  the  plugs  will 
spark  at  once  and  the  engine  will  start  without  cranking. 

Large  multiple-cylinder  engines  can  be  equipped  with 
a  compressed-air  starting-outfit.  This  consists  of  a  small 
air  compressor,  air  tank,  valves  and  piping.  When  it  is 
desired  to  start  the  engine  the  compressed  air  is  allowed 
to  flow  into  the  cylinder  in  which  the  piston  is  on  its 
down  stroke.  A  quick-opening  hand-operated  valve  is 
provided  for  each  cylinder. 

Where  electric  power  is  available  a  small  electric  motor 
furnishes  a  convenient  method  for  starting. 

Another  convenient  way  is  to  have  a  small  auxiliary 
engine  which  can  be  easily  started  by  hand  and  when 
running  furnishes  the  power  required  for  starting  the  large 
engine. 

187.  STOPPING. — In  stopping  an  engine  the  following 
operations  are  performed: 

Shut  off  the  fuel  supply;  turn  off  the  ignition  current; 
turn  off  the  lubricators;  turn  off  the  water;  throw  out 
the  clutch. 


172  GAS-ENGINE  THEORY  AND   DESIGN 

Open  the  relief -cock  so  that  the  compressed  charge  will 
not  rock  the  fly-wheel  back  and  forth  before  coming  to 
rest — when  connected  to  a  generator  this  rocking  will 
injure  the  brushes.  Another  reason  for  opening  the  relief- 
cock  is  to  clear  the  cylinders  of  all  explosive  charges  and 
thus  avoid  the  possibility  of  a  back  explosion  in  starting. 

188.  ENGINE  TROUBLES. — Assuming  that  the  design 
and  construction  are  good,  half  of  the  gas-engine  troubles 
are  caused  by  defective  installations  and  the  other  half 
by  careless  handling. 

Poor  foundations,  small  piping  with  many  elbows, 
improper  distance  from  the  line  shaft,  wrong  connections, 
dirt  and  obstacles  in  connections,  leaky  joints,  etc.,  are 
often  the  result  of  installing  an  engine  cheaply. 

If  the  engine  has  been  properly  installed  then,  in  order 
to  avoid  trouble,  the 

Ignition  apparatus  must  be  kept  in  good  order. 

Lubrication  should  receive  systematic  attention. 

All  water  and  oil  joints  must  be  tight,  the  oil  and  water 
must  be  filtered. 

All  parts  liable  to  become  loose  on  account  of  engine 
vibration  must  be  examined  at  regular  intervals. 

Bearings  must  be  examined  frequently.  Knocking  and 
pounding  are  caused  by  play  in  the  bearings.  To  deter- 
mine quickly  where  the  noise  may  be  the  use  of  the  stetho- 
scope has  been  suggested. 

Everything  in  connection  with  the  fuel  supply  from 
tank  to  combustion  chamber  must  be  in  good  order. 

Pistons  and  valves  must  not  be  leaky. 

When  pistons  and  valves  bind,  due  to  gumming  up  of 
lubricating  oil,  or  carbon  deposits,  a  little  kerosene  will 
quickly  loosen  the  parts. 

The  inlet  and  exhaust  passages  must  be  kept  clean. 

The  piping  must  be  so  arranged  that  any  moisture  which 


GAS-ENGINE   MANIPULATION 


173 


accumulates  can  be  drained  off.  All  parts  that  must  be 
cleaned  at  intervals,  or  adjusted,  must  be  arranged  so  that 
they  can  be  reached  handily. 

When  an  engine  is  operated  on  city  gas,  a  rubber  bag, 
and  sometimes  in  addition  a  pressure  regulator,  is  placed 
between  the  engine  and  gas  main  in  such  a  manner  that  the 
pulsations  of  the  engine  will  not  affect  lights  or  burners 
in  the  building  which  are  supplied  from  the  main.  A 


Diaphragm 


FIG.  129a. 

pressure  regulator  is  shown  in  Fig.  129a.    The  action  of 
this  is  obvious. 

When  an  engine  is  thoroughly  tested  out  in  the  shop 
in  the  first  place,  is  installed  properly  and  receives  intel- 
ligent care,  there  is  little  cause  for  trouble  after  instal- 
lation. In  order  to  obtain  good  results  a  gas  engine  must 
be  properly  looked  after,  the  same  as  a  steam  engine  and 
boiler,  or  any  other  machine  or  apparatus.  The  construe 
tion  and  operation  of  the  engine  should  be  studied  by  the 
man  who  takes  care  of  it,  and  this,  together  with  a  sys- 
tematic inspection,  will  overcome  all  ordinary  troubles. 
The  time  required  for  keeping  a  gas  engine  in  good  condition 
is  less  than  that  required  for  many  other  machines. 


CHAPTER  XX 

TESTING 

189.  The  object  of  testing  an  engine  is  to  determine 
its  power,  thermal  and  mechanical  efficiency  under  different 
loads,,  and  to  bring  out  and  remedy  defects  in  design, 
construction,  and  adjustment.  The  performance  of  each 
engine  (in  the  case  of  a  stock  engine)  should  equal  a  stand- 
ard determined  by  careful  experimental  work  in  the  way 
of  valve-setting,  timing  of  ignition,  etc.  When  a  new 
design  has  been  completed,  and  an  engine  built,  careful 
tests  will  show  where  improvements  can  be  made  in  both 
efficiency  and  general  design. 

Possibility  of  errors  and  wrong  conclusions  should  be 
eliminated  as  much  as  possible  by  the  use  of  proper  ap- 
paratus and  careful  observations. 

The  tests  should  be  continued  for  a  sufficient  length 
of  time  to  insure  the  engine's  running  in  the  same  manner 
under  actual  working  conditions.  For  example,  an  engine 
might  show  up  very  well  during  a  few  minutes'  run,  but 
after  it  has  been  installed  in  a  factory  and  run  for  several 
hours  it  may  overheat,  or  develop  other  troubles  which  a 
short  run  will  not  bring  out. 

The  test  should  include  runs  under  different  loads,  say: 
No  load. 
Quarter  load. 
Half  load. 
Three-quarter  load. 
Full  load. 
Overload. 

174 


TESTING  175 

The  test  should  bring  out  the  following: 
(a)  Energy  put  into  the  machine. 
(6)  Work  done  in  the  engine  cylinder. 

(c)  Outside  work  done  by  the  engine. 

(d)  Heat  losses. 

(e)  Mechanical  losses. 

190.  ENERGY  PUT  INTO  THE  ENGINE. — We  will  assume 
that  the  test  under  discussion  is  made  under  full  load  and 
continued  for  one  hour.     The  energy  put  into  the  engine 
equals  the  B.T.U.  per  pound  of  fuel  multiplied  by  the 
pounds  of  fuel  used.     The  heating  value  of  the  fuel  is  de- 
termined by  calorimeter  tests. 

191.  WTORK    DONE   IN   THE   ENGINE   CYLINDER. — This 
is  the  indicated  horse-power  computed  from  the  indicator 
diagram. 

The  thermal  efficiency  = 

Total  R.T.U.  X  B.T.U.  equivalent  of  I.H.-P. 
Total  B.T.U. 

The  principle  of  the  ordinary  form  of  indicator  suitable 
for  slow-speed  engines  has  already  been  described. 

192.  THE  MANOGRAPH. — The  ordinary  indicator  is  not 
suitable   for   high-speed   work,  and   the  manograph,    an 
indicator  of  special  form,  must  here  be  used.     By  means 
of  this  instrument  a  diagram  can  be  obtained  at  any  speed. 
Its  construction  is  as  follows: 

A  ray  of  light  enters  a  closed  box  through  a  pin-hole. 
This  ray  is  deflected  by  a  concave  mirror  which  concen- 
trates the  light  on  a  ground-glass  screen.  The  mirror 
has  both  a  horizontal  and  vertical  movement.  The 
horizontal  movement  is  produced  by  a  crank  operated 
by  the  engine  crankshaft.  The  vertical  movement  is 
produced  by  a  diaphragm  which  is  provided  with  a  tube 
connection  to  the  engine  combustion  chamber.  The 


176 


GAS-ENGINE  THEORY   AND   DESIGN 


Card  1. 


travel  of  the  piston  will,  therefore,  produce  a  horizontal 
movement  of  the  mirror,  while  the  pressure  in  the  cylinder 
produces  a  vertical  movement.  When  the  engine  is  run- 
ning, a  point  of  light 
travels  rapidly  over  the 
glass  screen,  and  the 
movement  is  so  fast  that 
there  appears  to  the  eye 
a  continuous  line  of 
light.  Permanent  rec- 
ords can  be  made  by 
photographing  the  light 
diagram. 

Some  manograph 
cards  from  a  Franklin 
automobile  engine  are 
given  herewith: 

Card  No.  1  was  taken 
from  cylinder  No.  3. 
H.-P.  lOf  at  700  R.  P.M. 
The  closed  lines  were 
obtained  by  cutting  out 
the  spark.  The  light 
line  over  the  main  ex- 
pansion line  shows  the 
increased  power  due  to 
a  completely  scavenging 
cylinder  being  fired 
after  missing  several  ex- 
plosions. 

Card  No.  2  was  taken 

from  cylinder  No.  2.     H.-P.  and  R.P.M.  the  same  as  above. 

Card  No.  3  was  taken  from  cylinder  No.  3  of  the  engine 

under  test.     H.-P.  developed,  14£,  at  1,000  R.P.M.     The 


Card 


TESTING  177 

rapid  drop  in  pressure  near  the  end  of  the  stroke  is  due 
to  the  auxiliary  exhaust. 

The  compression  pressure  is  60  Ibs.  gauge,  and  the  ex- 
plosion pressure  is  about  350  Ibs. 

This  engine  has  a  4£"  base,  4"  stroke. 

193.  EXPLOSION  RECORDER. — Another  valuable  instru- 
ment  for  gas-engine  testing   is  the  Mathot   continuous- 
explosion  recorder.     Here  a  paper  ribbon 

unwinds  from  one  drum  and  on  to  another 
drum.     The  drums  are  turned  by  clockwork 
and  the  paper  travels  at  a  certain  prede- 
termined speed.      The   explosions   are  re- 
corded by  a  pencil  set  in  motion  as  in  the  ordinary  in- 
dicator.    The  record  (see  Fig.  130)  shows  the  regularity 
and  time  of  the  explosions. 

By  means  of  either  of  these  two  instruments  it  can  be 
quickly  determined  whether,  in  a  multiple-cylinder  en- 
gine, each  cylinder  is  doing  its  share  of  the  work.  Faults 
can  be  corrected  and  the  result  of  changes  seen  at  once. 

194.  OUTSIDE  WORK  DONE  BY  THE  ENGINE. — This  is 
the  brake  horse-power,  or  the  power  which  the  engine  is 
capable  of  delivering. 

W  X  L  X  2  X  3.1416  X  R.P.M. 
33,000 

where  L  is  the  length  of  the  brake  arm  in  feet,  i.e.,  length 
from  centre  of  rotation  to  centre  line  of  scale  or  spring; 
W  is  the  pull  on  the  brake  arm  in  pounds. 

I.H.-P.xB.H.-P. 
Mechanical  efficiency  = j  „  p — 

The  difference  between  I.H.-P.  and  B.H.-P.  is  the  work 
done  in  overcoming  engine  friction. 

The  ordinary  dynamometer  and  pony  brake  are  too 
well  known  to  require  description  here.  For  high-speed 

12 


178 


GAS-ENGINE  THEORY   AND   DESIGN 


engines  a  fan  can  be  made  into  a  very  convenient  and 
satisfactory  dynamometer.  This,  of  course,  requires  no 
cooling  water. 

195.  DYNAMO    DYNAMOMETER. — This    is    an    ordinary 
direct-current  dynamo  so  arranged  that  its  field  swings 
in  ball  bearings.     An  adjustable  weight,  mounted  on  an 
arm  fastened  to  the  frame,  balances  the  magnetic  torque 
between  the  rotating  armature  and  the  field. 

B.H.-P.=  W  X  R.P.M.X  a  constant. 
The  constant  is  determined  by  dynamo  tests.     The  effi- 
ciency of  the  dynamo  does  not  enter  into  the  calculations. 
A  field  theostat  and  a  load  rheostat  complete  the  equipment. 

196.  HYDRAULIC  BRAKE. — There  are  several  forms  of 
the  hydraulic  brake,  but  Fig.  131  illustrates  the  principles 


v 


FIG.  131. 

involved.  Several  plates  are  fastened  to  the  casing  C, 
plates  B  are  fastened  to  the  shaft,  water  enters  at  A 
and  circulates  along  the  shaft.  As  the  shaft  revolves 
the  water  is  forced  between  the  plates,  which  are  close 
together,  and  is  thrown  against  the  inside  of  the  casing 
by  centrifugal  force.  The  water  drains  off  at  D.  The  pas- 
sage of  the  water  between  the  plates  causes  considerable 
friction  and  the  heat  so  generated  is  carried  off  by  the 
water.  As  the  plates  fastened  to  the  shaft  revolve  they 


TESTING  179 

tend,  of  course,  to  revolve  the  casing,  and  the  power  is 
measured  as  in  the  ordinary  brake.  The  brake  will  absorb 
more  or  less  power  according  to  the  adjustment  of  the 
valves  at  A  and  D. 

197.  HEAT  LOSSES. — These  include  the  following:  water- 
jacket  loss;  exhaust-gases  loss;   losses  due   to  imperfect 
combustion,  radiation,  etc. 

198.  WATER-JACKET  Loss. — The    heat    carried  off    by 
the  cooling  water  equals  the  weight  of  the  cooling  water 
X(t'—  t"),  where  t'  is  the  temperature  of  the  outgoing 
water  and  t"  is  the  temperature  of  the  incoming  water. 

199.  HEAT  LOST  IN  THE  EXHAUST. — The  heat  lost  in 
the  exhaust  gases  equals  the  weight  of  the  exhaust  gases 
in  pounds  X  their  specific  heat  X  their  absolute  tempera- 
ture. 

The  specific  heat  of  the  exhaust  gases  can  be  determined 
more  or  less  closely  by  analyzing  them. 

From  the  total  heat  lost  in  the  exhaust  must  be  sub- 
tracted the  heat  in  the  air  (or  air  and  gas)  supplied  per 
hour.  This  heat  equals  W  X  T  X  sp.  ht. 

The  heat  lost  in  the  exhaust  is  also  found  by  adding 
the  I.H.-P.  and  water-jacket  loss  and  subtracting  the  sum 
from  the  total  heat  supplied. 

Both  of  the  foregoing  methods  are  wrong,  in  that  they 
will  not  give  accurate  results.  In  the  first  case  the  weights 
exhaust  temperatures,  and  specific  heats  cannot  be  ac- 
curately determined,  and  in  the  second  case  the  exhaust 
is  charged  with  losses  due  to  incomplete  combustion, 
radiation,  etc. 

The  only  way  in  which  to  accurately  determine  the  heat 
lost  in  the  exhaust  is  to  pass  the  gases  through  a  condenser 
and  cool  them  to  the  original  temperature,  then  find  how 
much  heat  has  been  absorbed  by  the  condenser  water, 
making  due  allowance  for  the  heat  absorbed  by  the  metal. 


180 


GAS-ENGINE   THEORY   AND   DESIGN 


TESTING  181 

Assuming  that  there  has  been  no  leakage,  the  difference 
between  the  total  heat  supplied  and  the  I.H.-P.,  plus 
water-jacket  loss,  plus  exhaust  loss,  must  be  charged  to 
incomplete  combustion  and  radiation. 

The  testing  of  engines  at  definite,  intervals  is  apt  to  result 
in  a  considerable  saving  of  fuel  and  in  increase  in  power. 
Instances  are  on  record  where  the  thermal  efficiency  has 
been  increased  fully  30  per  cent  and  the  power  largely 
increased. 

200.  MECHANICAL  LOSSES. — This  includes  engine  fric- 
tion, leakage,  back  pressure  in  exhausting,  etc.  The  leak- 
age losses  may  become  very  large  if  pistons  and  valves 
do  not  fit  well. 

In  conclusion  some  indicator  diagrams  are  given  (Figs. 
132  to  135)  which  illustrate  the  effects  of  various  wrong 
conditions  in  the  gas  engine. 

The  efficiency  of  an  engine  under  ordinary  working 
conditions  will  fall  short  of  that  determined  by  experts 
in  testing  perfectly  adjusted  engines  under  the  best  pos- 
sible conditions,  and  this  must  be  borne  in  mind  in  figuring 
on  the  fuel  consumption  and  maximum  power  of  an  engine 
for  every-day,  and  perhaps,  unfavorable,  conditions. 

Tachometer  diagrams  should  be  also  taken  in  order  to 
determine  the  correct  fly-wheel  weight. 


CHAPTER     XXI 

DESIGNS 

201.  Marine  Engine.— The  first  design  (M-A,  M-B,  M-l 
to  M-57)  shown  is  that  of  a  small  canoe  or  boat  engine 
designed  by  the  author.  This  engine  is  compact  and  neat, 
has  a  3"  bore  and  3"  stroke,  and  at  1,000  R.P.M.  will 
develop  about  2^  H.-P.  The  following  notes  will  help 
in  the  study  of  this  design : 

The  crank-shaft  is  ample  in  diameter,  the  weight  of  the 
engine  complete  is  about  40  Ibs.,  the  fly-wheel  is  heavy 
enough  to  swing  a  12"  propeller  with  a  12"  or  15"  pitch. 
With  some  modifications  the  design  can  be  arranged  for  a 
four-cylinder  engine.  The  screw  threads  are  U.  S.  S. 
throughout;  no  pipe  is  used. 

The  ports  are  larger  than  is  usual  in  such  small  engines, 
the  gas  passages  are  as  direct  as  possible,  the  exhaust  port 
is  uncovered  while  the  piston  travels  the  last  f"  of  its 
down  stroke,  the  cylinder  inlet  port  is  uncovered  a  little 
later,  the  crank-case  inlet  port  is  uncovered  the  same  length 
of  time  as  the  exhaust  port.  Straight  passages  prevent 
wire-drawing  and  back  pressure  and  so  increase  the  power. 

The  lubricating  oil  is  carried  along  by  the  incoming  air 
and  lubricates  all  parts  in  the  crank-case  and  cylinder 
in  an  efficient  manner.  Twelve  drops  per  minute  is  suffi- 
cient for  good  lubrication. 

The  pump  is  held  in  place  by  a  small  reverse  clutch  (not 
shown).  These  clutches  are  equipped  with  ball  thrust 
bearings. 

182 


DESIGNS  183 

The  water  passes  from  the  jacket  into  the  muffler,  where 
it  sprays  on  top  of  the  exhaust  pipe,  thus  cooling  the 
exhaust  and  reducing  the  volume  of  the  exhaust  gases. 
A  shoulder  on  the  muffler  casting  prevents  any  water 
which  may  run  into  the  exhaust  pipe  from  blowing  back 
into  the  cylinder. 

A  three-terminal  coil  is  used  in  the  ignition  apparatus. 
The  wiring  plan  is  shown  in  M-57.  One  of  the  secondary 
terminals  is  connected  to  a  primary  terminal  inside  of  the 
coil.  The  return  wire  for  the  batteries  is  connected  to  the 
engine  frame  which  forms  a  ground  for  both  primary  and 
secondary  circuits. 

The  water  jacket  is  spun  from  22-gauge  copper,  then 
where  it  covers  bosses  on  cylinder  the  metal  is  raised  and 
holes  punched.  The  parts  that  screw  into  these  bosses  are 
all  fitted  with  shoulders  so  as  to  make  a  tight  joint.  The 
bottom,  where  the  ring  goes,  is  rolled  down  to  5.31"  and 
the  ring  shrunk  on. 

M-10  is  a  good  quality  bronze  casting. 

M-12  is  either  a  steel  forging  or  cut  and  turned  from  a  bar. 

M-16  is  made  up  of  two  aluminum  castings,  one-half 
having  a  shoulder  and  the  other  half  being  recessed  cor- 
respondingly. The  bushings  are  of  bronze.  The  carbu- 
reter castings  are  aluminum  or  brass.  The  float  is  ad- 
justed by  moving  up  and  down  on  pin  until  gasolene  level 
is  Ty  below  point  of  needle  valve.  The  muffler  castings 
are  aluminum. 

M-32  is  made  of  18-gauge  sheet  metal,  perforated  and  rolled. 

M-35  and  M-57.  Flexible  rubber  joint.  The  water- 
pump  parts  are  of  bronze. 

M-42.     Two  f"  steel  balls  forming  check  valves. 

M-49  is  a  bronze  casting. 

M-54  is  made  of  No.  20  steel  wire,  outside  diameter  of 
coil  I",  wound  four  turns  per  %". 


184 


GAS-ENGINE   THEORY   AND   DESIGN 


M-58  is  the  relief-cock. 

To  start  the  engine  open  the  carbureter  needle  valve 
one-half  turn,  open  the  relief  cock,  open  carbureter  butter- 
fly valve  one-half  turn,  set  commutator  for  a  spark  a  little 
past  the  dead  centre,  turn  over  the  fly-wheel  several  times 
until  there  is  an  explosion.  The  crank-case  is  now  filled 
with  an  explosive  mixture.  Close  the  relief-cock  and  start 


M-A. 


DESIGNS 


185 


the  engine.  A  two-cycle  engine  with  crank-case  compres- 
sion will  not  start  until  the  crank-case  is  filled  with  an  ex- 
plosive mixture.  When  the  engine  is  running  turn  on  the 
lubricating  oil,  adjust  carbureter,  and  commutator  for 
steady  running.  If  the  engine  is  stopped  after  having  run 
for  some  time,  it  can  be  restarted  by  one  turn  of  the  starting- 
handle. 


M-B. 


186 


GAS-ENGINE  THEORY  AND  DESIGN 


M-l.-Cylinder. 


DESIGNS 


187 


M-2. 


188 


GAS-ENGINE  THEORY   AND   DESIGN 


M-3.— Water-jacket  Ring.     F.A.O. 


DESIGNS 


189 


Holes  for  Ring 
Pins— No.  14  Wire 


190  GAS-ENGINE  THEORY   AND   DESIGN 


•f 


XT*  XT 


-H- 


..!_ 


j*-*1-* 


•2JS- 


M-7—  Piston  Pin.     F.A.O. 


M 


M-8.— Piston  Pin  Set  Screw. 


M-9. — Piston  Ring.     Three  wanted. 


DESIGNS 


191 


M-10.— Connecting-Rod. 


192 


GAS-ENGINE  THEORY   AND   DESIGN 


LLLL 


DESIGNS 


193 


M-13—  Fly-wheel. 


194  GAS-ENGINE  THEORY   AND   DESIGN 


Taper  tf  per  foot 
M-14.— F.A.O. 


DESIGNS 


J95 


Starting  Handle 


M-15. 


196  GAS-ENGINE  THEORY   AND   DESIGN 

i 
— f- 


M-16.— €rank-Case.     Right  and  left  casting. 


DESIGNS 


197 


I-16a. — Crank-Case  Studs.     Four  wanted  with  nuts. 


M-17. — Crank-Case  Bolt.     Two  wanted  with  nuts. 


M-18. — Crank-Case  Bolt.     Three  wanted  with  nuts. 


198  GAS-ENGINE  THEORY  AND   DESIGN 

j          ^  j'e  Hole  for  Grease  Cup 


M-19. — Main  Bearing  Bushing.     Two  wanted.     F.A.O. 


M-20.— Carbureter  Casting. 


DESIGNS 


199 


M-20a. — Carbureter  Cover. 


M-21.— Carbureter  Float. 


1 


M-22. — Carbureter  Cover  Screw.     Two  wanted. 


200  GAS-ENGINE   THEORY  AND   DESIGN 


M-23.— Carbureter  Fitting.     F.A.O. 


M-24. — Carbureter  Needle  Valve. 


DESIGNS 


201 


M-25.— Needle  Valve  Fitting. 


£^fjf£- 


M-26.— Needle  Valve  Seat. 


M-27. — Carbureter  Throttle  Valve. 


202 


GAS-ENGINE   THEORY  AND  DESIGN 


DESIGNS 


203 


M-30.— Muffler  Cover. 
I 


M-31.— Muffler  Cover  Bolt.     Two  wanted. 


M-32. — Muffler  Tube.     18  gauge — perforations  iV  wide. 


204 


GAS-ENGINE  THEORY  AND   DESIGN 


I 

M-33—  Exhaust  Fitting.    F.A.O. 


M-34.— Exhaust  Tube. 


Groove  for  Lubricant 


M-36.— Water  Pump  Eccentric.     F.A.O. 


DESIGNS 


205 


- 

M-36a.— W.  P.  Key. 


M-37.— Water  Pump  Eccentric  Ring. 


M-39.— Pump  Plunger.     F.A.O. 


206  GAS-ENGINE  THEORY   AND   DESIGN 


Drill  for  *  14  Wire  Pin 


M-40.— Pump  Pin. 


M-41. — Water  Pump  Casting. 


DESIGNS 


207 


M-43—  Pump  Cap.     F.A.O. 


M-44.— Pump  Cap.     F.A.O. 


208 


GAS-ENGINE  THEORY  AND   DESIGN 


M-45  and  M-56. — For  Pump  Casting  and  Commutator  Handle.     Three 
wanted. 


M-46.— Water  Inlet  Tube. 




K* 

•— 

Lizzie 

- 

1 

*| 

*  —  x-  —  > 

*t 

<-M- 

I 

M-47  and  M-48.— Pump  Floor  Plate.     F.A.O. 


DESIGNS 


209 


MM 

M-49. — Commutator  Handle. 


210  GAS-ENGINE  THEORY  AND   DESIGN 


Commutator  Bushings 
Fiber 


M-50. — Commutator  Bushings  Fiber.     Two  wanted. 


KnurL 


M-52.— €.  F.  Cap.     F.A.O. 


211 


i* «- 

M-51. — Commutator  Fitting.     F.A.O. 


M-53. — C.  Contact  pin. 
Steel-hardened. 


M-55.— C.  Contact  Screw. 


M-57—  Wiring  Diagram. 


212  GAS-ENGINE  THEORY   AND   DESIGN 


202.  Horizontal  Engine.— Pl&tes  S-36  to  S-68  show 
assembly  views  and  details  of  a  small  horizontal  four-cycle 
gas  or  gasolene  engine,  which  has  been  partly  re-designed 
by  the  author.  These  details  are  so  complete  that  only 
a  few  words  of  explanation  are  required. 

The  machine  develops  about  ^  H.-P.  running  at  750 
R.P.M.  The  water  is  allowed  to  boil  off.  The  pump 
furnishes  gasolene  to  a  vaporizer  (not  shown)  which  is 
provided  with  an  overflow  pipe.  The  governor,  in  which 
four  steel  balls  depress  a  plate,  operates  a  throttle  valve 
in  the  vaporizer.  The  piston  is  lubricated  by  means  of 
an  oil  cup  (sec  S-36)  mounted  on  a  stem  which  is  screwed 
into  a  tapped  hole  in  the  cylinder. 

The  few  small  parts  not  shown  can  easily  be  bought  in 
the  market.  The  engine  runs  very  smoothly  and  illustrates 
some  excellent  and  interesting  principles  in  design.  The 
arrangement  of  base,  sub-base,  cylinder,  camshaft,  gover- 
nor, etc.,  is  closely  followed  out  in  the  design  of  many  large 
horizontal  gas  engines. 


DESIGNS 


213 


214  GAS-ENGINE  THEORY   AND   DESIGN 


DESIGNS 


215 


L>16 


GAS-ENGINE   THEORY   AND   DESIGN 


DESIGNS 


217 


218 


GAS-ENGINE  THEORY  AND   DESIGN 


DESIGNS 


219 


220 


GAS-ENGINE  THEORY   AND   DESIGN 


DESIGNS 


221 


222 


GAS-ENGINE  THEORY   AND   DESIGN 


DESIGNS 


224 


GAS-ENGINE   THEORY   AND   DESIGN 


DESIGNS 


225 


226  GAS-ENGINE  THEORY    AND   DESIGN 


J  \ 


DESIGNS 


227 


lt_.'SJ50_R.P.:.I. 


H f-'£-> 

S-51.— Spiral  Gear.     F.A.O. 


228 


GAS-ENGINE   THEORY    AND    DESIGN 


S-52.— Cylinder  Head  Cover. 


DESIGNS 


229 


230  GAS-ENGINE  THEORY   AND   DESIGN 


fc 

*                                                U, 

1                                                 ! 

• 

1 

1 

1 

"i 

1  I 

; 

L 

— 

V 

j>;j& 

i 

il«-                   -                     iv"                            >k        i3/" 

—  > 

S-54.— C.  I.  Cylinder  Liner. 


S-58. — Governor  Assembly. 


DESIGNS 


231 


232 


GAS-ENGINE   THEORY   AND   DESIGN 


g      ! 

?_i 


-"<;• 


•% 


DESIGNS 


?33 


234 


GAS-ENGINE   THEORY   AND    DESIGN 


"l-f  H 


DESIGNS 


235 


S-60. — Ball  Support. 


S-60. — Governor  Cap.     C.R.  Steel.     F.A.O. 


236 


GAS-ENGINE   THEORY  AND   DESIGN 


S-61.— Governor  Plate. 


S-62. — Governor  Plate. 


DESIGNS 


237 


S-63.— Cylinder  Head  Assembly 


238 


GAS-ENGINE   THEORY   AND    DESIGN 


DESIGNS 


239 


k — : 


240 


GAS-ENGINE  THEORY   AND    DESIGN 


DESIGNS 


241 


242  GAS-ENGINE  THEORY   AND   DESIGN 

T# 

III 


APPENDIX 


TESTS. — Some  tests  of  Warren  gas  engines  are  herewith 
given  as  submitted  by  the  builders: 

SYNOPSIS    OF    THE    RESULTS    OBTAINED     FROM 

THE  NONPAREIL  CORK  WORKS'   ENGINE 

AND    PRODUCER 


Date 

Hours 
Run 

Hours. 
Stand. 

Fuel 
Run 

Con- 
sumed 
Stand. 

Output 
H.-P. 
Hrs. 

LOAD  FACTOR 

Consump- 
tion 
Per 
H.-P.  Hr. 

Max. 

Min. 

Avg. 

Jan.  22.. 
"  23.  . 

ff 

s* 

2100 
2147 

350 
360 

832.3 
1516.9 

53% 

SP3 

35.48% 
7.09 

42.7% 
68.95 

2.5lb. 
1.41 

"  24.. 
"  25.. 
"  26.. 

lit 

8i 

13 
12f 
13 

1435 

l.VJS 
706 

355 
355 
355 

1501 
1421 
730.1 

89.3 
80.8 
53.6 

9.85 
11.3 
20.5 

67.2 
66.6 
42.95 

.92 
1.12 
.967 

The  load  factors  were  based  upon  the  engine  rating  of  200  B.H.-P. 
The  B.H.-P.  developed  was  computed  from  30  min.  readings  of  am- 
meter and  volt-meter,  allowing  15  per  cent  loss  for  the  belt  and  gen- 
erator. 

The  coal  charged  to  the  producer  was  weighed  and  the  magazine 
was  kept  full  at  all  times,  so  that  amount  of  coal  lost  over  night  could 
be  very  accurately  determined  by  cleaning  the  fires  in  the  morning 
and  then  filling  the  magazine  to  the  top. 


Average  Ibs.  of  coal  per  B.H.-P.  per  hr. . 

"        load   factor 

"        stand-by   loss 

"  "        "    per  H.-P.  based 

on  producer  rating  of  300  H.-P  .  . 
243 


1.33  Ibs. 
58.7    per  cent. 
27.2    Ibs.  coal  per  hr. 

9 . 1    Ibs.  per  100  hrs. 


1M-1 


APPENDIX 


GAS  ENGINE,  GAS  PRODUCER  AND  No.  3  DYNAMO 
Five  days  run,  June  10,  11,  12,  13  and  14,  1907,  11  hrs.  per  day. 


DATE 

Avg.  Volts 

Avg.  Am. 

Lbs.  Pea  Coal 

Gals.  Oil 

June  10  

225 

516 

2,450 

2 

"        11  

225 

524 

2,800 

2 

"       12  

225 

485 

2,800 

2 

"       13  

225 

492 

2,800 

2 

"       14  

225 

456 

2,100 

2 

Total  

1,125 

2,473 

12,950 

10 

Average   

225 

495 

2,590 

2 

Average  Volts  225         Average  Amperes  .  .  .       495 

Total  K.W.  Hours  6115         Total  B.H.-P.  Hrs.  de- 

veloped by  engine  10,200 

1  K.W.  Hour=  1.341  El.  II.-P.  Hours.  With  88  per  cent  efficiency  of 
dynamo  and  92  per  cent  efficiency  of  belt,  1  K.W.  Hour=1.66  Brake 
H.-P.  Hours  developed  by  the  engine. 


Cost  of  anthracite  pea  coal  used,  per  gross  ton.  . . . 
Cost  of  gas-engine  oil  used,  per  gallon 


$3.91 
.21 


Analysis 
of  Coal. 

Moisture 

Volatilcs 

Sulphur 

Fixed  Carbon. . 
Ash.  .  . 


Coal  Consumption  Including 
Stand-by  Losses  of  400  Ib. 
per  Twenty-four  Hours. 


Heating  Value 
of  Coal  12,657 
B.T.U.  per  Ib. 

1.15  per  cent 

4.35  per  cent 

.     0.60  per  cent 

.   77 . 35  per  cent 

.    16.55  per  cent 

100.00  per  cent 


Coal  Consump- 
tion, Charge- 
able to  Engine, 
Exclusive  of 
Stand-by 
Losses. 


Total 12950        Ib.      10950 

Per  K.W.  Hourat  switchboard         2.121b.  1.7! 


Per  B.H.-P.  Hour  at  engine.  .       1.27  Ib. 


1.071b. 


APPENDIX  245 


Operating  Costs, 

Operating  Costs  Including  Chargeable  to  En- 

Stand-by  Losses.  gine,  Less  Stand-by 


Coal  per  K.W.  Hour 0.305c  0.258c 

Coal  per  B.H.-P.  Hour 0. 183c  0. 154c 

Oil  per  K.W.  Hour 0.035c  0.035c 

Oil  per  B.H.-P.  Hour 0.021c  0.021c 

Labor  per  K.W.  Hour 0.515c  0.515c 

Labor  per  B.H.-P.  Hour  ....  0 . 310c  0 . 310c 

Total  per  K.W.  Hour 0 . 855c  0 . 608c 

Total  per  B.H.-P.  Hour 0 . 514c  0 . 485c 

The  figures  above  are  for  a  load  of  about  80  per  cent  of  the  full  capac- 
ity of  the  engine,  and  about  60  per  cent  of  the  capacity  of  the  producer. 
At  this  load  the  engine  developed  one  B.H.-P.  on  13,500  B.T.U., meas- 
ured by  coal  consumed  in  the  producer,  and  taking  the  efficiency  of 
the  producer  as  75  per  cent  the  engine  used  10,000  B.T.U.  per  B.H.-P. 
Hour,  or  an  amount  equal  to  about  9,500  B.T.U.  at  full  load  on  engine. 


REPORT  OF  TEST  OF  ONE  (1)  250  H.-P.  WARREN  TANDEM 

GAS  ENGINE  AT  POWER   HOUSE  OF  THE  CITIZENS 

GAS  &  ELECTRIC  COMPANY,  LORAIN,  OHIO, 

FEBRUARY  28ra  AND  29TH,   1908 

ENGINE 

Two-Cylinder  Warren  Tandem  Gas  Engine,  built  by  Struthers-Wells 
Company,  Warren,  Pa. 

Diameter  of  Cylinder 19  in. 

Stroke 25  " 

Diameter  of  Piston  Rod 4J" 

Speed 170  R.P.M. 

GENERATOR 

Engine  was  belt-connected  to  150  K.W.  Westinghouse,  3-phase,  60- 
cycle,  A.C.  Generator,  39.5  amperes,  2,200  volts  at  600  R.P.M. 

TEST 

The  electrical  output  of  the  generator  was  transmitted  partly  to  a 
water  rheostat  and  partly  to  the  lighting  system  of  the  Company. 


246  APPENDIX 

The  test  was  started  at  5:00  P.M.  February  28th,  1908,  the  load 
being  20  amperes  per  phase  at  2,400  volts.  The  load  was  gradually 
increased  until  5:30  P.M.,  at  which  time  engine  was  carrying  load  of 
45  amperes  per  phase,  and  at  this  time  the  Westinghouse  Three-Cylin- 
der Vertical  Engine  was  started,  synchronized,  and  run  in  parallel  with 
the  Warren  Tandem  Engine.  When  the  two  engines  were  operating, 
the  load  on  the  Warren  Tandem  engine  averaged  35  amperes  and  2,300 
volts  per  phase.  This  load  condition  continued  until  9:30  P.M.,  at 
which  time  the  load  on  the  Warren  Engine  gradually  decreased  to  27 
amperes  at  2,300  volts  per  phase.  At  10:00  P.M.  the  use  of  the  West- 
inghouse Engine  was  discontinued  and  the  entire  load  carried  by 
Warren  Tandem  Engine,  the  load  at  that  time  averaging  48  amperes 
per  phase  at  2,300  volts. 

The  night  load  from  10:00  P.M.  of  February  28th  until  5:30  A.M. 
February  29th  averaged  45  amperes  at  2,300  volts  per  phase,  equal  to 
179  K.V.A.  Assuming  at  that  time  a  power  factor  of  the  whole  cir- 
cuit as  low  as  0.70,  we  would  have  an  average  of  125.5  K.W.  at  the 
switchboard,  or  an  average  of  201  B.H.-P.  developed  by  the  engine  for 
seven  hours  and  thirty  minutes,  or  a  total  of  1,507.5  B.H.-P.  hours. 
The  gas  consumption  for  this  period  of  time  was  12,650  cubic  feet; 
12,650  divided  by  1,507.5  equals  8.4-cubic  feet  of  gasper  B.H.-P.  hour. 

READINGS  OF  ENGINE  AND  GENERATOR  PERFORMANCE  AT  10:35  P.M., 
FEBRUARY  28ra 

Load  on  generator,  46.3  amperes,  2,300  volts  equal  to  184  K.V.A. 
Mean  effective  pressure  in  engine  cylinders,  79  pounds. 
Engine  speed,  172  R.P.M. 
Engine  load  equal  to  238  I.H.-P. 

Assuming  the  mechanical  efficiency  of  the  engine  at  this  load  to  be 
88  per  cent,  we  would  have  209  B.H.-P.  at  the  engine,  or  132  K.W.  at 

132 
the  generator;  the  power  factor  being  T^J-=  0.716. 

AVERAGE  READING  FROM  11:15  A.M.,  FEBRUARY  29TH  TO  12:15  P.M. 

Average  load  on  generator,  43  amperes,  2,470  volts,  equal  to  184 
K.V.A. 

Mean  effective  pressure  on  engine  cylinders,  105.8  pounds. 

Engine  speed,  170  R.P.M. 

Engine  load  equal  to  316  I.H.-P. 

Assuming  the  mechanical  efficiency  of  the  engine  at  this  load  to  be 
88  per  cent,  we  would  have  281  B.H.-P.  at  the  engine,  or  176  K.W.  on 

the  generator;   the  power  factor  being  -=-  =  .957. 

Io4 


APPENDIX 


247 


The  gas  consumed  from  11:15  A.M.  until  12: 15  P.M.  was  2,290  cubic 
feet;  2,290  divided  by  281  equals  8.1  cubic  feet  of  gas  per  B.H.-P.  hour. 


SPEEDS  AT  VARIOUS  LOADS 

After  3:00  P.M.  the  load  was  repeatedly  changed  in  order  to  ascer- 
tain the  speed  regulation,  which  was  ascertained  to  be  as  follows: 


Variation  from  170  R.P.M.2.35% 
Variation  from  170  R.P.M.  1.76% 
Variation  from  170  R.P.M.  1.18% 
Variation  from  170  R.P.M.  0.00% 
Variation  from  170  R.P.M.  0.59% 


Full  load  speed 170  R.P.M. 

Speed  at  no  load 174  R.P.M. 

Speed  at  i  load 173  R.P.M. 

Speed  at  *  load 172  R.P.M. 

Full  load  speed 170  R.P.M. 

Speed  at  overload  (320 

B.H.P.) 169  R.P.M. 

Speed  variation  under  constant  load  less  than  plus  or  minus  0.25 
R.P.M.  equals  0.15  per  cent. 

The  engine  showed  itself  capable  of  carrying,  without  undue  heat- 
ing or  any  signs  of  distress,  a  load  on  the  generator  equal  to  54  am- 
peres at  2,150  volts,  equal  to  201  K.W.  The  M.E.P.  developed  in  the 
engine  cylinders  at  this  load  was  119.5  pounds  at  169  R.P.M.,  equal 
to  320  B.H.-P.  or  355  I.H.-P. 

The  mechanical  efficiency  of  the  engine  when  carrying  this  load  was 

equal  to  !;—  =  90  per  cent. 
GOO 

The  determination  of  the  amount  of  power  developed  by  the  engine 
as  shown  by  switchboard  readings  is  based  on  the  following  data: 

1  K.W.  =  1.34  B.H.-P.,  exclusive  of  losses  at  generator  or  through 
transmission  of  power  by  belt. 

Assuming  the  average  generator  efficiency  at  91  per  cent  for  differ- 
ent loads  specified,  and  belt  efficiency  (the  belt  being  new)  at  92  per 
cent,  1  K.W.  at  the  switchboard  would  be  equivalent  to  1.6  B.H.-P. 
developed  by  the  engine. 

Attached  hereto  are  indicator  cards,  numbers  1  to  4,  inclusive: 

No.  1.  Showing  M.E.P.  in  engine  cylinders  67  pounds,  speed  at  time 
of  taking  card  172  R.P.M.  Generator  load,  34.3  amperes  per  phase  at 
2,400  volts. 

No.  2.  Showing  M.E.P.  in  engine  cylinders  79  pounds,  speed  at  time 
of  taking  card  171  R.P.M.  Generator  load,  46.3  amperes  per  phase  at 
2,300  volts. 

No.  3.  Showing  M.E.P.  in  engine  cylinders  106  pounds,  speed  at  time 
of  taking  card  170  R.P.M.  Generator  load,  43  amperes  per  phase  at 
2,450  volts. 

No.  4.  Showing  M.E.P.  in  engine  cylinders  119.5  pounds,  speed  at 
time  of  taking  card  169  R.P.M.  Generator  load,  54  amperes  per  phase 
at  2,460  volts. 


248 


APPENDIX 


#    «    00 

id 

H 

y 

ii  H 

ol  cC 

SS 


II   II 


«S5 
d 


^ffi 


APPENDIX 


249 


APPENDIX 


§    £       §5 
EH    82s 


I 


OH   ffi 


APPENDIX 


251 


INDEX 


PAR. 

Adiabatic  expansion 83 

Advancing  of  spark 112-120 

Advantages  of  the  gas  engine  .  95 

Air,  composition  of 47 

cooling 127 

required  for  combustion  48-52 

required  per  H.-P 144 

Alcohol 64 

production  of 64 

Altitude,  effect  of 146 

Anthracite  coal 58 

Anti-freezing  solutions 130 

Applications  of  the  gas  engine  15 

Ash 45 

Atmosphere 73, 147 

Atom 34 

Automobile  statistics 15 

Auxiliary  exhaust 134,  186 

Avogadro's  law 53 

Babbitt,  brass,  bronze 178 

Balancing 157-157a 

Banki  engine 128 

Bearings 177-178 

Beau  de  Rochas  cycle 11 

Benzol 716 

Bituminous  coal 59 

Bolts,  strength  of 166 

Bore  and  stroke 145 

Brake  horse-power 194 

Brake,  hydraulic 196 

Brayton  cycle 10 

British  thermal  unit 23-147 


Calorie 

Calorific  power. 
Calorimetry. . . . 

Cams 

Carbureters. .  .  . 
Carnot  cycle.  .  . 


. . .  147 
44-52a 
. ...  72 
. ..  179 
. ..  147 
..  90 


Centigrade  thermometer 30 

Centrifugal  force 180 

gas  cleaner .   69 

Chamber,  combustion 104 

Change,  physical 34 

chemical 34 

Chemical  action 34 

Chemical    reactions    in    pro- 
ducers       71 

Classification,  general 3 

Clerk  engine 12 

Coal 57-59 

production  of 57 

Coef.  linear  expansion 33a 

Coke 66 

Combustion 34-35,  55 

complete 41 

compound 34 

incomplete 42,  103 

of  a  compound 52 

speed  of 46 

Compression 88,  99,  142 

two-stage 89 

Conduction 21 

Connecting-rod.  150,  154,  157,  173 

Conservation  of  energy 147 

Convection 21 

Cooling 124 

Counterbalancing 157a 

Crank 175 

pin 176 

shaft 174 

Crosshead 171q 

Curve,  PVn 149 

Cycle 4-26 

two-stroke 6 

four-stroke 7 

Cylinder  head : 1 65 

Cylinder  volume 144 

Cylinders 163 

Cylinders,  arrangements  of .  .    161 
253 


254 


INDEX 


PAR. 

PAR. 

Daimler  engine                              13 

Gas,  illuminating  

66 

Density  33 

laws  of  

...     73 

Design  of  marine  engine  201 

natural  

...     65 

Design  of  parts  159 

oil  water  

...     68 

Design  of  stationary  engine.  .   201 
Diagrams,   four-cycle  engine.  158 

producer  
specific  heat  of  

...     70 
...     91 

Diesel  cycle  14 

volume  of  

...     91 

Dilution  of  explosive  mixtures  102 

water  

...     67 

Dissociation  55 

Gasolene  

...     63 

Distillates,  petroleum  60 

Gay-Lussac,  law  <.f  

...     76 

Distillation  60 

Governor,  cut-off'  

.    Ill 

Distribution  120 

function  of  

.    .    108 

Dynamometer,  dynamo  195 

quality  

...    109 

Economy  94 

quantity  
throttle  

...    110 
.  .  .    110 

Efficiency                               92  189 

Element  34 

Heat,  definition  of  

16 

Energy  147 

and  power  units  

...    147 

available  87 

engine  

1 

intrinsic  79 

insensible  

...     25 

put  into  engine  190 

latent  

.    27-29 

Exhaust  131,  133,  134 

losses  

...    197 

losses  199 

mechanical  equivalent 

of.      24 

using  heat  of  93 

sensible  and  insensible 

..  .      25 

Expansion  18,  33a,  81 

sources  of  

...      20 

adiabatic  83 

specific  26 

,  86,  91 

isothermal                                 82 

theory  of  

17 

PVn                                                 87 

transfer  of  

21 

Explosion  39,  81 

Heat  unit  

.  .  .      23 

Explosive  mixtures  OS,   K)2 

Historical  

.   8-14a 

Explosive  pressures  ....    148,  149 

Hollow  shaft  

.  .  .    174 

Horizontal  vs.  vertical  .  .  . 

...    137 

Fahrenheit  thermometer.  ...      30 

Horse-power  

...    148 

Firing  cylinders,  order  of.  ...    120 

Hydrocarbons  

.  .  .      60 

Flame  36 

Flashing-point  60 

Ignition  37, 

113-119 

Fly-wheel  180 

Indicated  H.-P  

74,  191 

Foot-pound  24 

Indicator  

...      74 

Forces  acting  in  gas  engine  .  .    148 

diagram  73, 

148-149 

Foundations  182 

Induction  coil  

.  .  .    118 

Four-stroke  cycle  7 

Inertia  

155-156 

cycle  engine  diagrams.  .  .    158 
Frames  162 

Installation  of  gas  engine 
Isometric  lines  

.    184 

...      84 

Fuels  43,  56,  61,  71«,  100 

Isopiestic  lines  

...      85 

enriching  of  716 

Isothermal  expansion.  .  .  . 

...     82 

Fusion  28,  30,  33a 

Jump  spark  

.  .  .    117 

Gallon                                             147 

Gas   changes  in                              75 

Kerosene  

62 

blast-furnace  69 

Kilowatt  

.  .  .    147 

coal  66 

Kinetic  energy  

.  .  .    147 

cleaning  of  69-70 

engine  2 

Latent  heat  

.      27 

fuel,  properties  of  7  la 

Laws,  combining  of  

.  .  .      78 

INDEX 


255 


PAR. 

Laws  of  gases 73 

of  thermodynamics 33 

Lenoir  cycle 9 

3,  exhaust 199 

heat 197 

mechanical. . .  .  .  200 


PAH. 

Producer,      chemical      reac- 
tions in 70 

gas 71 

Products  of  combustion 53 

Pyrometry 32 


water-jacket  

198 

Radiation  

.  .  .  .      22 

Lubrication  and  lubricants  .  . 

178 

Recorder,  explosion  

.  .  .  .    193 

Reliability 

93 

M 

Re'sume 

la 

Make-and-break  ignition 

119 

Rotation,  direction  of.  .  . 

'.'.'.'  161a 

Manipulation  of  gas  engine  .  . 
Manograph  
Marine  exhaust  

184 
192 
133 

Scavenging  
Selection  of  type  

.    101 
.  ...    136 

Mariotte,  law  of  

77 

Shaft,  crank  

.  ...    174 

IfiO 

Side  thrust 

153    169 

Mean  effective  pressure  
Mechanical  losses  

1OU 

73 
200 

Single-acting  
Slider-velocity  diagram  . 

138,  139 
.  ...    151 

equivalent  of  heat  
Media  used  in  heat  engines.  . 
Metric  units  

24 
96 

147 

Small  units 

.  ...    140 

.  .  .  .      38 
.  .  .  .      20 

Smoke 

Sources  of  heat  

Mixing  valves  

105 

Spark  advancing  

.  ...    112 

Mixtures,  mechanical  

34 

Spark-plug  

117,  122 

Molecule 

34 

Specific  gravity  

.    33a-60 

Specific  heat  

,..86,91 

Nomenclature,  chemical  
Non-conductor  

34 

22 

Speed  
Spontaneous  combustion 

...        3 
.  .  .  .     40 

Springs  

...    179 

Offset  cylinders  

152a 

Starting  devices  
of  gas  engine  

.  ..    186 
...    185 

Oil,  fuel  

61 

States  of  matter 

29 

Oilers,  mechanical  
Operation,  general  principles 

178 

Statistics,  automobile 

15 

alcohol  

...     64 

of  

5 

coal  

...     57 

gas  engine  

...    14a 

Passages,  valve  

179 

motor  boat  

...      15 

Petroleum  

60 

petroleum  

70 

distillates  

61 

Stopping  of  gas  engine  .  .  . 

...    187 

production  of  
Physical    properties    of    ma- 

60 

Storage  battery  
Strength  of  materials  .... 

...    121 
...    183 

terials  

33a 

Stroke  and  bore  

...    145 

properties  of  liquids  

33a 

Studs  

...    166 

Piston  

168 

Stuffing-boxes  

...    178 

pin  

170 

Suction  gas-producer  .... 

....   70 

rings  

172 

rod  

171 

Tachometer  

.    181 

speed  
Planimeter  

143 

73 

Tangential-effort  diagram  ...    152 
Temperature  19,  31,  54 

Potential  energy  

147 

Testing  

...    189 

Power  141, 

147 

Tests  of  gas  engines  

...   201 

Preheating  

107 

Theoretical  temperatures. 

..  .      54 

Pressure,  absolute  

73 

Thermal  conductivity.  .  .  . 

...   33a 

regulator  

188 

Thermodynamics,  laws  of 

...     33 

Principles  of  operation  

5 

Thermometer,  air  

...     31 

256 


INDEX 


Thermometry 

Timers 

Troubles,  engine 

Two-cycle  vs.  four-cycle. 

Two-stroke  cycle 

Types  of  heat  engines.  .  . 


Unit,  heat 
power 

Valve  cages. . 
Valves 
Vapor 

Vaporization . 
Vaporizers. . . 


..   23, 


PAR. 
30 
120 
188 
136 
6 
97 

147 
147 


PAR. 

Volumes  of  gases 91 

of  products  of  combustion     53 


167 

. . 105, 179 

33 

29 

.    106 


Water,  cooling 

impurities  in 

injection 

jacket 

Watt 

Weight,  atomic 

of  materials 

Work. 

delivered 

in  engine  cylinder 

Zero,  absolute 


125,  129 

....  130 

....  128 

164,  198 

....  147 

....  34 

....  33a 

147 

194 

191 

.  31 


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This  book  is  DUE  on  the  last  date  stamped  beloiv 


NOV   3  0   1953 
JAN   1  8    195 
MAR   2 
DEC  6 


1954 


mn.  i 


Form  L9-100m-9,'52(A3105)444 


